GIFT   ©F 
MICHAEL  REESE 


TUNNELING: 


A    PRACTICAL   TREATISE 


BY 

CHARLES    PRELINI,  C.  E, 

WITH  ADDITIONS  BY 

CHARLES   S.  HILL,  C.E. 

ASSOCIATE  EDITOR  "ENGINEERING  NEWS" 


DIAGRAMS  AND   ILLUSTRATIONS 


NEW   YORK: 

D.  VAN    NOSTRAND   COMPANY 

23  MURRAY  AND  27  WARREN  STS. 
1901 


COPYRIGHT,  IQOI, 
D.  VAN  NOSTRAND  COMPANY. 


TYPOGRAPHY  BY 
C.  J.  PETERS  &  SON, 

BOSTON,  MASS. 
U.S.A. 


PREFACE 


IN  his  work  at  Manhattan  College  the  writer  found  himself 
confronted  by  the  fact  that  there  were  but  two  books  on  Tun- 
neling in  the  English  language,  neither  of  which  he  could  rec- 
ommend as  text  books  for  his  pupils.  Drinker's  tunneling  is 
a  splendid  reference  book,  and  may  be  consulted  with  advan- 
tage by  any  engineer,  but  it  is  too  voluminous  and  expensive 
to  be  suitable  for  the  beginner.  Simins's  Practical  Tunnel- 
ing is  a  magnificent  exposition  of  the  English  method  of 
tunneling,  but  it  is  too  old  for  anyone  who  looks  for  the  most 
modern  methods  of  tunneling,  as  the  art  has  progressed  greatly 
since  Mr.  Simms's  death.  The  additions  introduced  by  Mr. 
D.  K.  Clarke,  although  they  convey  an  excellent  idea  of  the 
manner  of  excavating  long  tunnels  like  the  Mont  Cenis  and 
St.  Gothard,  fail  in  what  may  be  called  real  practical  value, 
viz.,  to  explain  to  engineers  and  contractors  the  vaiiou's  meth- 
ods of  driving  tunnels  of  ordinary  dimensions  through  different 
soils. 

Having  thus  felt  the  want  of  a  book  of  convenient  size  and 
moderate  price,  the  author  began  to  enlarge  the  notes  of  his 
lectures  for  publication.  The  general  purpose  of  the  book 
which  has  resulted  is  to  explain  all  the  operations  that  are  re- 
quired hi  tunneling,  and  then  illustrate  by  suitable  examples 
the  actual  application  of  these  methods  in  practice.  Formulas 
and  difficult  calculations  have  been  avoided,  the  book  being 
simply  descriptive,  and  the  text  well  illustrated,  so  that  it  can 
be  easily  understood  by  students  and  others  unfamiliar  with 
tunneling  work.  This  work  of  preparation  has  been  very  diffi- 
cult to  the  writer  owing  to  the  fact  that,  being  a  foreigner,  the 

iii 


IV  PREFACE 

language  was  not  fully  mastered,  and,  having  been  but  a  few 
years  in  this  country,  he  was  not  familiar  with  what  had  been 
accomplished  here  in  previous  years.  The  latter  fault  was 
remedied  as  far  as  possible  by  a  careful  consultation  of  the 
Transactions  of  the  American  Society  of  Civil  Engineers,  the 
volumes  of  Engineering  News,  and  of  other  periodicals,  with 
all  of  which  the  writer  made  very  free  use.  The  writer  has 
received  much  assistance  from  his  friends  in  the  preparation  of 
the  manuscript,  and  takes  this  opportunity  to  thank  them  for 
their  trouble  and  encouragement.  It  is  his  wish,  however,  to 
give  special  thanks  to  Mr.  Charles  S.  Hill,  Associate  Editor 
Engineering  News,  whose  suggestions  and  criticisms  led  to  many 
changes  and  additions,  and  extended  the  scope  of  the  book. 

CHAKLES  PRELINI. 
February,  1901. 


CONTENTS 


PAGE 

INTRODUCTION  — THE  HISTORICAL  DEVELOPMENT  OF  TUNNEL  BUILD- 
ING       ix 

CHAPTER 

I.     PRELIMINARY    CONSIDERATIONS  ;    CHOICE   BETWEEN    A    TUNNEL 
AND   AN   OPEN  CUT  ;   METHOD  AND  PURPOSE   OF  GEOLOGICAL 

SURVEYS .          1 

II.     METHODS  OF  DETERMINING  THE  CENTER   LINE   AND  FORMS  AND 

DIMENSIONS  OF  CROSS-SECTION 91 

III.  EXCAVATING    MACHINES    AND  ROCK    DRILLS  ;    EXPLOSIVES    AND 

BLASTING 19 

IV.  GENERAL  METHODS   OF   EXCAVATION  ;    SHAFTS  ;   CLASSIFICATION 

OF  TUNNELS      .  * 32 

V.     METHODS  OF  TIMBERING  OR  STRUTTING  TUNNELS 43 

VI.     METHODS  OF  HAULING  IN  TUNNELS 55 

VII.     TYPES    OF    CENTERS   AND   MOLDS    EMPLOYED   IN   CONSTRUCTING 

TUNNEL  LININGS  OF  MASONRY 62 

VIII.      METHODS  OF  LINING  TUNNELS   .     .     .-   .     .  -.     .     *    .     ...       68 
IX.     TUNNELS   THROUGH  HARD  ROCK  ;  GENERAL  DISCUSSION  ;  EXCA- 
VATION BY  DRIFTS;  MONT  CENIS  TUNNEL 79 

X.     TUNNELS     THROUGH    HARD    ROCK    (continued) ;    THE    SIMPLON 

TUNNEL  ..*.."... 94 

XI.     TUNNELS  THROUGH    HARD    ROCK    (continued) ;    EXCAVATION   BY 

DRIFTS  ;  ST.  GOTHARD  TUNNEL  ;  BUSK  TUNNEL 114 

XII.     REPRESENTATIVE     MECHANICAL     INSTALLATIONS     FOR    TUNNEL 

WORK     .,.-....*..» 124 

XIII.  EXCAVATING  TUNNELS  THROUGH  SOFT  GROUND  ;    GENERAL  DIS- 

CUSSION ;  THE  BELGIAN  METHOD 133 

XIV.  THE     GERMAN    METHOD    OF    EXCAVATING    TUNNELS     THROUGH 

SOFT  GROUND  ;  BALTIMORE  BELT-LINE  TUNNEL 145 

XV.     THE  FULL-SECTION  METHOD  OF  TUNNELING  ;  ENGLISH  METHOD, 

AUSTRIAN  METHOD 156 

v 


VI 


CONTENTS 


CHAPTER  PAGE 

XVI.     SPECIAL  TREACHEROUS  GROUND   METHOD  ;    ITALIAN   METHOD  ; 

QUICKSAND  TUNNELING  ;  PILOT  METHOD 167 

XVII.     OPEN- CUT    TUNNELING     METHODS;     TUNNELS    UNDER     CITY 

STREETS  ;  B.OSTON  SUBWAY  AND  NEW  YORK  RAPID  TRANSIT,     180 
XVIII.     SUBMARINE  TUNNELING  ;    GENERAL   DISCUSSION  ;  THE    SEVERN 

TUNNEL 201 

XIX.     SUBMARINE    TUNNELING    (continued)  ;    THE  EAST    RIVER   GAS 

TUNNEL  ;  THE  VAN  BUREN  STREET  TUNNEL,  CHICAGO     .     .     208 
XX.     SUBMARINE  TUNNELING  (continued)  ;  THE  MILWAUKEE  WATER- 
WORKS TUNNEL 280 

XXI.     SUBMARINE  TUNNELING  (continued)  ;  THE  SHIELD  SYSTEM  .     .     242 
XXII.     ACCIDENTS  AND  REPAIRS  IN  TUNNELS  DURING  AND  AFTER  CON- 
STRUCTION        266 

XXIII.  RELINING  TIMBER-LINED  TUNNELS  WITH  MASONRY     ....     280 

XXIV.  THE  VENTILATION  AND   LIGHTING   or   TUNNELS    DURING   CON- 

STRUCTION „     .     .     .     „ 290 

XXV.     THE  COST  OF  TUNNEL  EXCAVATION,  AND  THE  TIME  REQUIRED 

FOR  THE  WORK 300 

INDEX  .  309 


IWTEODUOTIOU" 


THE  HISTORICAL   DEVELOPMENT   OF   TUNNEL 

BUILDING. 

A  TUNNEL,  defined  as  an  engineering  structure,  is  an  artificial 
gallery,  passage,  or  roadway  beneath  the  ground,  under  the  bed 
of  a  stream,  or  through  a  hill  or  mountain.  The  art  of  tunnel- 
ing has  been  known  to  man  since  very  ancient  times.  A  The- 
ban  king  on  ascending  the  throne  began  at  once  to  drive  the 
long,  narrow  passage  or  tunnel  leading  to  the  inner  chamber  or 
sepulcher  of  the  rock-cut  tomb  which  was  to  form  his  final 
resting-place.  Some  of  these  rock-cut  galleries  of  the  ancient 
Egyptian  kings  were  over  750  ft.  long.  Similar  rock-cut  tun- 
neling work  was  performed  by  the  Nubians  and  Indians  in 
building  their  temples,  by  the  Aztecs  in  America,  and  in  fact 
by  most  of  the  ancient  civilized  peoples. 

The  first  built-up  tunnels  of  which  there  are  any  existing 
records  were  those  constructed  by  the  Assyrians.  The  vaulted 
drain  or  passage  under  the  southeast  palace  of  Nimrud,  built  by 
Shalmaneser  II.  (860-824  B.C.),  is  in  all  essentials  a  true  soft- 
ground  tunnel,  with  a  masonry  lining.  A  much  better  exam- 
ple, however,  is  the  tunnel  under  the  Euphrates  River,  which 
may  quite  accurately  be  claimed  as  the  first  submarine  tunnel 
of  which  there  exists  any  record.  It  was,  however,  built  under 
the  dry  bed  of  the  river,  the  waters  of  which  were  temporarily 
diverted,  and  then  turned  back  into  their  normal  channel  after 
the  tunnel  work  was  completed,  thus  making  it  a  true  sub- 
marine tunnel  only  when  finished.  The  Euphrates  River  tun- 
nel was  built  through  soft  ground,  and  was  lined  with  brick 

ix 


X  INTRODUCTION 

masonry,  having  interior  dimensions  of  12  ft.  in  width  and  15 
ft.  in  height. 

Only  hand  labor  was  employed  by  these  ancient  peoples  in 
their  tunnel  work.  In  soft  ground  the  tools  used  were  the 
pick  and  shovels,  or  scoops.  For  rock  work  they  possessed  a 
greater  range  of  appliances.  Research  has  shown  that  among 
the  Egyptians,  by  whom  the  art  of  quarrying  was  highly  de- 
veloped, use  was  made  of  tube  drills  and  saws  provided  with 
cutting  edges  of  corundum  or  other  hard,  gritty  material.  The 
usual  tools  for  rock  work  were,  however,  the  hammer,  the  chisel, 
and  wedges ;  and  the  excellence  and  magnitude  of  the  works 
accomplished  by  these  limited  appliances  attest  the  unlimited 
time  and  labor  which  must  have  been  available  for  their  ac- 
complishment. 

The  Romans  should  doubtless  rank  as  the  greatest  tunnel 
builders  of  antiquity,  in  the  number,  magnitude,  and  useful 
character  of  their  works,  and  in  the  improvements  which  they 
devised  in  the  methods  of  tunnel  building.  They  introduced 
fire  as  an  agent  for  hastening  the  breaking  down  of  the  rock, 
and  also  developed  the  familiar  principle  of  prosecuting  the 
work  at  several  points  at  once  by  means  of  shafts.  In  their 
use  of  fire  the  Romans  simply  took  practical  advantage  of  the 
familiar  fact  that  when  a  heated  rock  is  suddenly  cooled  it 
cracks  and  breaks  so  that  its  excavation  becomes  comparatively 
easy.  Their  method  of  operation  was  simply  to  build  large 
fires  in  front  of  the  rock  to  be  broken  down,  and  when  it  had 
reached  a  high  temperature  to  cool  it  suddenly  by  throwing 
water  upon  the  hot  surface.  The  Romans  were  also  aware 
that  vinegar  affected  calcareous  rock,  and  in  excavating  tunnels 
through  this  material  it  was  a  common  practice  with  them  to 
substitute  vinegar  for  water  as  the  cooling  agent,  and  thus  to 
attack  the  rock  both  chemically  and  mechanically.  It  is  hardly 
necessary  to  say  that  this  method  of  excavation  was  very  severe 
on  the  workmen  because  of  the  heat  and  foul  gases  generated. 
This  was,  however,  a  matter  of  small  concern  to  the  builders* 


INTRODUCTION  XL 

since  the  work  was  usually  performed  by  slaves  and  prisoners 
of  war,  who  perished  by  thousands.  To  be  sentenced  to  labor 
on  Roman  tunnel  works  was  thus  one  of  the  severest  penalties 
to  which  a  slave  or  prisoner  could  be  condemned.  They*  were 
places  of  suffering  and  death  as  are  to-day  the  Spanish  mercury 
mines. 

Besides  their  use  of  fire  as  an  excavating  agent,  the  Romans 
possessed  a  very  perfect  knowledge  of  the  use  of  vertical  shafts 
in  order  to  prosecute  the  excavation  at  several  different  points 
simultaneously.  Pliny  is  authority  *  for  the  statement  that  in 
the  excavation  of  the  tunnel  for  the  drainage  of  Lake  Fucino 
forty  shafts  and  a  number  of  inclined  galleries  were  sunk  along 
its  length  of  3^  miles,  some  of  the  shafts  being  400  ft.  in 
depth.  The  spoil  was  hoisted  out  of  these  shafts  in  copper 
pails  of  about  ten  gallons'  capacity  by  windlasses. 

The  Roman  tunnels  were  designed  for  public  utility.  Among 
those  which  are  most  notable  in  this  respect,  as  well  as  for 
being  fine  examples  of  tunnel  work,  may  be  mentioned  the  nu- 
merous conduits  driven  through  the  calcareous  rock  between 
Subiaco  and  Tivoli  to  carry  to  Rome  the  pure  water  from  the 
mountains  above  Subiaco.  This  work  was  done  under  the 
Consul  Marcius.  The  longest  of  the  Roman  tunnels  is  the  one 
built  to  drain  Lake  Fucino,  as  mentioned  above.  This  tunnel 
was  designed  to  have  a  section  of  6  ft.  X  10  ft. ;  but  its  actual 
dimensions  are  not  uniform.  It  was  driven  through  calcareous 
rock,  and  it  is  stated  that  30,000  men  were  employed  for  eleven 
years  in  its  construction.  The  tunnels  which  have  been  men- 
tioned, being  designed  for  conduits,  were  of  small  section ;  but 
the  Romans  also  built  tunnels  of  larger  sections  at  numerous 
points  along  their  magnificent  roads.  One  of  the  most  notable 
of  these  is  that  which  gives  the  road  between  Naples  and  Poz- 
zuoli  passage  through  the  Posilipo  hills.  It  is  excavated 
through  volcanic  tufa,  and  is  about  3000  ft.  long  and  25  ft. 
wide,  with  a  section  of  the  form  of  a  pointed  arch.  In  order 

*  "  Tunneling,"  Encly.  Brit.,  1889,  vol.  xxiii.,  p.  623. 


Xll  INTRODUCTION" 

to  facilitate  the  illumination  of  this  tunnel,  its  floor  and  roof 
were  made  gradually  converging  from  the  ends  toward  the 
middle ;  at  the  entrances  the  section  was  75  ft.  high,  while  at 
the  center  it  was  only  22  ft.  high.  This  double  funnel-like 
construction  caused  the  rays  of  light  entering  the  tunnel  to 
concentrate  as  they  approached  the  center,  and  thus  to  improve 
the  natural  illumination.  The  tunnel  is  on  a  grade.  It  was 
probably  excavated  during  the  time  of  Augustus,  although 
some  authorities  place  its  construction  at  an  earlier  date. 

During  the  Middle  Ages  the  art  of  tunnel  building  was 
practiced  for  military  purposes,  but  seldom  for  the  public  need 
and  comfort.  Mention  is  made  of  the  fact  that  in  1450  Anne 
of  Lusignan  commenced  the  construction  of  a  road  tunnel 
under  the  Col  di  Tenda  in  the  Piedmontese  Alps  to  afford 
better  communication  between  Nice  and  Genoa ;  but  on  account 
of  its  many  difficulties  the  work  was  never  completed,  although 
it  was  several  times  abandoned  and  resumed.  For  the  most 
part,  therefore,  the  tunnel  work  of  the  Middle  Ages  was  in- 
tended for  the  purposes  and  necessities  of  war.  Every  castle 
had  its  private  underground  passage  from  the  central  tower  or 
keep  to  some  distant  concealed  place  to  permit  the  escape  of 
the  family  and  its  retainers  in  case  of  the  victory  of  the  enemy, 
and,  during  the  defense,  to  allow  of  sorties  and  the  entrance 
of  supplies. 

The  tunnel  builders  of  the  Middle  Ages  added  little  to  the 
knowledge  of  their  art.  Indeed,  until  the  17th  century  and 
the  invention  of  gunpowder  no  practical  improvement  was 
made  in  the  tunneling  methods  of  the  Romans.  Engravings 
of  mining  operations  in  that  century  show  that  underground 
excavation  was  accomplished  by  the  pick  or  the  hammer  and 
chisel,  and  that  wood  fires  were  lighted  at  the  ends  of  the 
headings  to  split  and  soften  the  rocks  in  advance.  Although 
gunpowder  had  been  previously  employed  in  mining,  the  first 
important  use  of  it  in  tunnel  work  was  at  Malpas,  France, 
in  1679-81,  in  the  tunnel  for  the  Languedoc  Canal.  This 


INTRODUCTION  Xlll 

tunnel  was  510  ft.  long,  22  ft.  wide,  and  29  ft.  high,  and  was 
excavated  through  tufa.  It  was  left  unlined  for  seven  years, 
and  then  was  lined  with  masonry. 

With  the  advent  of  gunpowder  and  canal  building  the  first 
strong  impetus  was  given  to  tunnel  building,  in  its  modern 
sense,  as  a  commercial  and  public  utilitarian  construction,  since 
the  days  of  the  Roman  Empire.  Canal  tunnels  of  notable 
size  were  excavated  in  France  and  England  during  the  last 
half  of  the  17th  century.  These  were  all  rock  or  hard-ground 
tunnels.  Indeed,  previous  to  1800  the  soft-ground  tunnel  was 
beyond  the  courage  of  engineer  except  in  sections  of  such 
small  size  that  the  work  better  deserves  to  be  called  a  drift  or 
heading  than  a  tunnel.  In  1803,  however,  a  tunnel  24  ft. 
wide  was  excavated  through  soft  soil  for  the  St.  Quentin  Canal 
in  France.  Timbering  or  strutting  was  employed  to  support 
the  walls  and  roof  of  the  excavation  as  fast  as  the  earth  was 
removed,  and  the  masonry  lining  was  built  closely  following  it. 
From  the  experience  gained  in  this  tunnel  were  developed  the 
various  systems  of  soft-ground  subterrannean  tunneling  since 
employed. 

It  was  by  the  development  of  the  steam  railway  >  however, 
that  the  art  of  tunneling  was  to  be  brought  into  its  present 
prominence.  In  1820-26  two  tunnels  were  built  on  the  Liver- 
pool &  Manchester  Ry.  in  England.  This  was  the  beginning 
of  the  rapid  development  which  has  made  the  tunnel  one  of 
the  most  familiar  of  engineering  structures.  The  first  railway 
tunnel  in  the  United  States  was  built  on  the  Alleghany  & 
Portage  R.R.  in  Pennsylvania  in  1831-33 ;  and  the  first  canal 
tunnel  had  been  completed  about  13  years  previously  (1818-21) 
by  the  Schuylkill  Navigation  Co.,  near  Auburn,  Pa.  It  would 
be  interesting  and  instructive  in  many  respects  to  follow  the 
rise  and  progress  of  tunnel  construction  in  detail  since  the  con- 
struction of  these  earlier  examples,  but  all  that  may  be  said 
here  is  that  it  was  identical  with  that  of  the  railway. 

The  art  of   tunneling  entered   its  last  and  greatest  phase 


XIV  INTRODUCTION 

with  the  construction  of  the  Mont  Cenis  tunnel  in  Europe  and 
the  Hoosac  tunnel  in  America,  which  works  established  the 
utility  of  machine  rock-drills  and  high  explosives.  The  Mont 
Cenis  tunnel  was  built  to  facilitate  railway  communication 
between  Italy  and  France,  or  more  properly  between  Pied- 
mont and  Savoy,  the  two  parts  of  the  kingdom  of  Victor 
Emmanuel  II.,  separated  by  the  Alps.  It  is  7.6  miles  long, 
and  passes  under  the  Col  di  Frejus  near  Mont  Cenis.  Som- 
meiller,  Grattoni,  and  Grandis  were  the  engineers  of  this  great 
undertaking,  which  was  begun  in  1857,  and  finished  in  1872. 
It  was  from  the  close  study  of  the  various  difficulties,  the  great 
length  of  the  tunnel,  and  the  desire  of  the  engineers  to  finish 
it  quickly,  that  all  the  different  improvements  were  developed 
which  marked  this  work  as  a  notable  step  in  the  advance  of 
the  art  of  tunneling.  Thus  the  first  power-drill  ever  used  in 
tunnel  work  was  devised  by  Sommeiller.  In  addition,  com- 
pressed air  as  a  motive  power  for  drills,  aspirators  to  suck  the 
foul  air  from  the  excavation,  air  compressors,  turbines,  etc., 
found  at  Mont  Cenis  their  first  application  to  tunnel  construc- 
tion. This  important  role  played  by  the  Mont  Cenis  tunnel 
in  Europe  in  introducing  modern  methods  had  its  counterpart 
in  America  in  the  Hoosac  tunnel  completed  in  1875.  In  this 
work  there  were  used  for  the  first  time  in  America  power  rock- 
drills,  air  compressors,  nitro-glycerine,  electricity  for  firing 
blasts,  etc. 

There  remains  now  to  be  noted  only  the  final  development 
in  the  art  of  soft-ground  submarine  tunneling,  namely,  the  use 
of  the  shield  and  metal  lining.  The  shield  was  invented  and 
first  used  by  Sir  Isambard  Brunei  in  excavating  the  tunnel 
under  the  River  Thames  at  London,  which  was  begun  in  1825, 
and  finished  in  1841.  In  1869  Peter  William  Barlow  used  an 
iron  lining  in  connection  with  a  shield  in  driving  the  second 
tunnel  under  the  Thames  at  London.  From  these  inventions 
has  grown  up  one  of  the  most  notable  systems  of  tunneling 
now  practiced,  which  is  commonly  known  as  the  shield  system. 


INTRODUCTION  XV 

In  closing  this  brief  review  of  the  development  of  modern 
methods  of  tunneling,  to  the  presentation  of  which  the  re- 
mainder of  this  book  is  devoted,  mention  should  be  made  of 
a  form  of  motive  power  which  promises  many  opportunities  for 
development  in  tunnel  construction.  Electricity  has  long  been 
employed  for  blasting  and  illuminating  purposes  in  tunnel 
work.  It  remains  to  be  extended  to  other  uses.  For  hauling 
and  for  operating  certain  classes  of  hoisting  and  excavating 
machinery  it  is  one  of  the  most  convenient  forms  of  power 
available  to  the  engineer.  Its  successful  application  to  rock- 
drills  is  another  promising  field.  For  operating  ventilating 
fans  it  promises  unusual  usefulness. 


TUNNELING 


CHAPTER  I 

PRELIMINARY   CONSIDERATIONS.      CHOICE    BE- 
TWEEN   A    TUNNEL    AND    OPEN    CUT. 
GEOLOGICAL    SURVEYS 


CHOICE  BETWEEN  A  TUNNEL  AND  AN  OPEN  CUT 

WHEN  a  railway  line  is  to  be  carried  across  a  range  of 
mountains  or  hills,  the  first  question  which  arises  is  whether 
it  is  better  to  construct  a  tunnel  or  to  make  such  a  detour  as 
will  enable  the  obstruction  to  be  passed  with  ordinary  surface 
construction.  The  answer  to  this  question  depends  upon  the 
comparative  cost  of  construction  and  maintenance,  and  upon 
the  relative  commercial  and  structural  advantages  and  disad- 
vantages of  the  two  methods.  In  favor  of  the  open  road  there 
are  its  smaller  cost  and  the  decreased  time  required  in  its  con- 
struction. These  mean  that  less  capital  Avill  be  required,  and 
that  the  road  will  sooner  be  able  to  earn  something  for  its 
builders.  Against  the  open  road  there  are  :  its  greater  length 
and  consequently  its  heavier  running  expenses;  the  greater 
amount  of  rolling-stock  required  to  operate  it ;  the  heavy  ex- 
pense of  maintaining  a  mountain  road ;  and  the  necessity  of 
employing  larger  locomotives,  with  the  increased  expenses  which 
they  entail.  In  favor  of  the  tunnel  there  are :  the  shortening 
of  the  road,  with  the  consequent  decrease  in  the  operating 
expenses  and  amount  of  rolling-stock  required ;  the  smaller  cost 


.84746 


2  TUNNELING 

of  maintenance,  owing  to  the  protection  of  the  track  from  snow 
and  rain  and  other  natural  influences  causing  deterioration ; 
and  the  decreased  cost  of  hauling  due  to  the  lighter  grades. 
Against  the  tunnel,  there  are  its  enormous  cost  as  compared 
with  an  open  road  and  the  great  length  of  time  required  to 
construct  it. 

To  determine  in  any  particular  case  whether  a  tunnel  or  an 
open  road  is  best,  requires  a  careful  integration  of  all  the  factors 
mentioned.  It  may  be  asserted  in  a  general  way,  however,  that 
the  enormous  advance  made  in  the  art  of  tunnel  building  has 
done  much  to  lessen  the  strength  of  the  principal  objections  to 
tunnels,  namely,  their  great  cost  and  the  length  of  time  required 
for  their  construction.  Where  the  choice  lies  between  a  tunnel 
or  a  long  detour  with  heavy  grades  it  is  sooner  or  later  almost 
always  decided  in  favor  of  a  tunnel.  When,  however,  the  con- 
ditions are  such  that  the  choice  lies  between  a  tunnel  or  a 
heavy  open  cut  with  the  same  grades  the  problem  of  deciding 
between  the  two  solutions  is  a  more  difficult  one. 

It  is  generally  assumed  that  when  the  cut  required  will  have 
a  vertical  depth  exceeding  60  ft.  it  is  less  expensive  to  build 
a  tunnel  unless  the  excavated  material  is  needed  for  a  nearby 
embankment  or  fill.  This  rule  is  not  absolute,  but  varies 
according  to  local  conditions.  For  instance,  in  materials  of 
rigid  and  unyielding  character,  such  as  rock,  the  practical  limit 
to  the  depth  of  a  cut  goes  far  beyond  that  point  at  which  a 
tunnel  would  be  more  economical  according  to  the  above  rule. 
In  soils  of  a  yielding  character,  on  the  other  hand,  the  very 
flat  slope  required  for  stability  adds  greatly  to  the  cost  of 
making  a  cut. 

It  may  be  noted  in  closing  that  the  same  rule  may  be  em- 
ployed in  determining  the  location  of  the  ends  of  the  tunnel, 
for  assuming  that  it  is  more  convenient  to  excavate  a  tun- 
nel than  an  open  cut  when  the  depth  exceeds  60  ft.,  then 
the  open  cut  approaches  should  extend  into  the  mountain-  or 
hill-sides  only  to  the  points  where  the  surface  is  60  ft.  above 


CHOICE    BETWEEN    A    TUNNEL    AND    AN    OPEN    CUT 

grade,  and  there  the  tunnel  should  begin.  If,  therefore,  we 
draw  on  the  longitudinal  profile  of  the  tunnel  a  line  parallel  to 
the  plane  of  the  tracks,  and  60  ft.  above  it,  this  line  will  cut 
the  surface  at  the  points  where  the  open-cut  approaches  should 
cease  and  the  tunnel  begin.  This  is  a  rule-of-thumb  determi- 
nation at  the  best,  and  requires  judgment  in  its  use.  Should 
the  ground  surface,  for  example,  rise  only  a  few  feet  above  the 
60  ft.  line  for  any  distance,  it  is  obviously  better  to  continue 
the  open  cut  than  to  tunnel. 

THE   METHOD  AND  PURPOSE   OF  GEOLOGICAL  SURVEYS 

When  it  has  been  decided  to  build  a  tunnel,  the  first  duty 
of  the  engineer  is  to  make  an  accurate  geological  survey  of 
the  locality.  From  this  survey  the  material  penetrated,  the 
form  of  section  and  kind  of  strutting  to  be  used,  the  best  form 
of  lining  to  be  adopted,  the  cost  of  excavation,  and  various 
other  facts,  are  to  be  deduced.  In  small  tunnels  the  geological 
knowledge  of  the  engineer  should  enable  him  to  construct  a 
geological  map  of  the  locality,  or  this  knowledge  may  be  had 
in  many  cases  by  consulting  the  geological  maps  issued  by  the 
State  or  general  government  surveys.  When,  however,  the 
tunnel  is  to  be  of  great  length,  it  may  be  necessary  to  call  in 
the  assistance  of  a  professional  geologist  in  order  to  reconstruct 
accurately  the  interior  of  the  mountain  and  thereby  to  ascer- 
tain beforehand  the  different  strata  and  materials  to  be 
excavated,  thus  obtaining  the  data  for  calculating  both  the 
time  and  cost  of  excavating  the  tunnel. 

The  geological  survey  should  enable  the  engineer  to  deter- 
mine, (1)  the  character  of  the  material  and  its  force  of  cohe- 
sion, (2)  the  inclination  of  the  different  strata,  and  (3)  the 
presence  of  water. 

Character  of  Material.  —  The  character  of  the  material  through 
which  the  proposed  tunnel  will  penetrate  is  best  ascertained 
by  means  of  diamond  rock-drills.  These  machines  bore  an 


4  TUNNELING 

annular  hole,  and  take  away  a  core  for  the  whole  depth  of  the 
boring,  thus  giving  a  perfect  geological  section  showing  the 
character,  succession,  and  exact  thickness  of  the  strata.  By 
making  such  borings  at  different  points  along  the  center  line 
of  the  projected  tunnel,  and  comparing  the  relative  sequence 
and  thickness  of  the  different  strata  shown  by  the  cores,  the 
geological  formation  of  the  mountain  may  be  determined  quite 
exactly.  Where  it  is  difficult  or  impracticable  to  make  dia- 
mond drill  borings  on  account  of  the  depth  of  the  mountain 
above  the  tunnel,  or  because  of  its  inaccessibility,  the  engineer 
must  resort  to  other  methods  of  observation. 

The  present  forms  of  mountains  or  hills  are  due  to- 
weathering,  or  the  action  of  the  destructive  atmospheric  influ- 
ences upon  the  original  material.  From  the  manner  in  which 
the  mountain  or  hill  has  resisted  weathering,  therefore,  may  be 
deduced  in  a  general  way  both  the  nature  and  consistency  of 
the  materials  of  which  it  is  composed.  Thus  we  shall  gener- 
ally find  mountains  or  hills  of  rounded  outlines  to  consist 
of  soft  rocks  or  loose  soils,  while  under  very  steep  and  crested 
mountains  hard  rock  usually  exists.  To  the  general  knowl- 
edge of  the  nature  of  its  interior  thus  afforded  by  the  ex- 
terior form  of  the  mountain,  the  engineer  must  add  such 
information  as  the  surface  outcroppings  and  other  local  evi- 
dences permit. 

For  the  purposes  of  the  tunnel  builder  we  may  first  classify 
all  materials  as  either,  (1)  hard  rock,  (2)  soft  rock,  or  (3). 
soft  soil. 

Hard  rocks  are  those  having  sufficient  cohesion  to  stand 
vertically  when  cut  to  any  depth.  Many  of  the  primary  rocksr 
like  granite,  gneiss,  feldspar,  and  basalt,  belong  to  this  class, 
but  others  of  the  same  group  are  affected  by  the  atmosphere, 
moisture,  and  frost,  which  gradually  disintegrate  them.  They 
are  also  often  found  interspersed  with  pyrites,  whose  well- 
known  tendency  to  disintegrate  upon  exposure  to  air  intro- 
duces another  destructive  agency.  For  these  reasons  we  may 


CHOICE   BETWEEN    A   TUNNEL   AND   AN    OPEN    CUT  i> 

divide  hard  rocks  into  two  sub-classes ;  viz.,  hard  rocks  un- 
affected by  the  atmosphere,  and  those  affected  by  it.  This 
distinction  is  chiefly  important  in  tunneling  as  determining 
whether  or  not  a  lining  will  be  required. 

Soft  rocks,  as  the  term  implies,  are  those  in  whicti  the  force 
of  cohesion  is  less  than  in  hard  rocks,  and  which  in  consequence 
offer  less  resistance  to  attacks  tending  to  break  down  their 
original  structure.  They  are  always  affected  by  the  atmosphere. 
Sandstones,  laminated  clay  shales,  mica-schists,  and  all  schistose 
stones,  chalk  and  some  volcanic  rocks,  can  be  classified  in  this 
group.  Soft  rocks  require  "to  be  supported  by  timbering  during 
excavation,  and  need  to  be  protected  by  a  strong  lining  to 
exclude  the  air,  and  to  support  the  vertical  pressures,  and 
prevent  the  fall  of  fragments. 

Soft  soils  are  composed  of  detrital  materials,  having  so  little 
cohesion  that  they  may  be  excavated  without  the  use  of 
explosives.  Tunnels  excavated  through  these  soils  must  be 
strongly  timbered  during  excavation  to  support  the  verti- 
cal pressure  and  prevent  caving ;  and  they  also  always  require 
a  strong  lining.  Gravel,  sand,  shale,  clay,  quicksand,  and  peat 
are  the  soft  soils  generally  encountered  in  the  excavation  of 
tunnels.  Gravels  and  dry  sand  are  the  strongest  and  firmest ; 
shales  are  very  firm,  but  they  possess  the  great  defect  of  being 
liable  to  swell  in  the  presence  of  water  or  merely  by  exposure 
to  the  air,  to  such  an  extent  that  they  have  been  known  to 
crush  the  timbering  built  to  support  them.  Quicksand  and 
peat  are  proverbially  treacherous  materials.  Clays  are  some- 
times firm  and  tenacious,  but  when  laminated  and  in  the 
presence  of  water  are  among  the  most  treacherous  soils. 
Laminated  clays  may  be  described  as  ordinary  clays  altered 
by  chemical  and  mechanical  agencies,  and  several  modifications 
of  the  same  structure  are  often  found  in  the  same  locality. 
They  are  composed  of  laminae  of  lenticular  form  separated  by 
smooth  surfaces  and  easily  detached  from  each  other.  Lami- 
nated clays  generally  have  a  dark  color,  red,  ocher  or  greenish 


6  TUNNELING 

blue,  and  are  very  often  found  alternating  with  strata  of 
stiatites  or  calcareous  material.  For  purposes  of  construction 
they  have  been  divided  into  three  varieties. 

Laminated  clays  of  the  first  variety  are  those  which  'alter- 
nate with  calcareous  strata  and  are  not  so  greatly  altered  as 
to  lose  their  original  stratification.  Laminated  clays  of  the 
second  variety  are  those  in  which  the  calcareous  strata  are 
broken  and  reduced  to  small  pieces,  but  in  which  the  former 
structure  is  not  completely  destroyed  ;  the  clay  is  not  reduced 
to  a  humid  state.  Laminated  clays  of  the  third  variety 
are  those  in  which  the  clay  by  the  force  of  continued  disturb- 
ance, and  in  the  presence  of  water,  has  become  plastic. 
Laminated  clays  are  very  treacherous  soils ;  quicksand  and 
peat  may  be  classed,  as  regards  their  treacherous  nature, 
among  the  laminated  clays  of  the  third  variety. 

Inclination  of  Strata Knowing  the  inclination  of  the 

strata,  or  the  angle  which  they  make  with  the  horizon,  it  is 
easy  to  determine  where  they  intersect  the  vertical  plane  of  the 
tunnel  passing  through  the  center  line,  thus  giving  to  a  certain 
extent  a  knowledge  of  the  different  strata  which  will  be  met 
in  the  excavation,.  On  the  inclination  of  the  strata  depend  : 
(1)  The  cost  of  the  excavation  ;  the  blasting,  for  instance,  will 
be  more  efficient  if  the  rocks  are  attacked  perpendicular  to  the 
stratification;  (2)  The  character  of  the  timbering  or  strut- 
ting ;  the  tendency  of  the  rock  to  fall  is  greater  if  the  strata 
are  horizontal  than  if  they  are  vertical ;  (3)  The  character  and 
thickness  of  the  lining;  horizontal  strata  are  in  the  weakest 
position  to  resist  the  vertical  pressure  from  the  load  above 
when  deprived  of  the  supporting  rock  below,  while  vertical 
strata,  when  penetrated,  act  as  a  sort  of  arch  to  support  the 
pressure  of  the  load  above.  The  foregoing  remarks  apply 
only  to  hard  or  soft  rock  materials. 

In  detrital  formations  the  inclination  of  the  strata  is  an 
important  consideration,  because  of  the  unsymmetrical  pres- 
sures developed.  In  excavating  a  tunnel  through  soft  soil 


CHOICE    BETWEEN   A   TUNNEL    AND   AN    OPEN    CUT  7 

whose  strata  are  inclined  at  30°  to  the  horizon,  for  instance, 
the  tunnel  will  cut  these  strata  at  an  angle  of  30°.  By  the 
excavation  the  natural  equilibrium  of  the  soil  is  disturbed, 
and  while  the  earth  tends  to  fall  and  settle  on  bath  sides 
at  an  angle  depending  upon  the  friction  and  cohesion  of  the 
material,  this  angle  will  be  much  greater  on  one  side  than  on 
the  other  because  of  the  inclination  of  the  strata;  and  hence 
the  prism  of  falling  earth  on  one  side  is  greater  than  on  the 
other,  and  consequently  the  pressures  are  different,  or  in 
other  words,  they  are  unsymmetrical.  These  unsymmetrical 
pressures  are  usually  easily  taken  care  of  as  far  as  the  lining 
is  concerned,  but  they  may  cause  serious  cave-ins  and  badly 
distort  the  strutting.  Caving-iii  during  excavation  may  be 
prevented  by  cutting  the  materials  according  to  their  natural 
slope ;  but  the  distortion  of  the  strutting  is  a  more  serious 
problem  to  handle,  and  one  which  oftentimes  requires  the 
utmost  vigilance  and  care  to  prevent  serious  trouble. 

Presence  of  Water.  —  An  idea  of  the  likelihood  of  finding 
water  in  the  tunnel  may  be  obtained  by  studying  the  hydro- 
graphic  basin  of  the  locality.  From  it  the  source  and  direction 
of  the  springs,  creeks,  ravines,  etc.,  can  be  traced,  and  from 
the  geological  map  it  can  be  seen  where  the  strata  bearing 
these  waters  meet  the  center  line.  Not  only  ought  the  surface 
water  to  be  attentively  studied,  but  underground  springs,  which 
are  frequently  encountered  in  the  excavation  of  tunnels,  re- 
quire careful  attention.  Both  the  surface  and  underground 
waters  follow  the  pervious  strata,  and  are  diverted  by  im- 
pervious strata.  Rocks  generally  may  be  classed  as  im- 
pervious ;  but  they  contain  crevices  and  faults,  which  often 
allow  water  to  pass  through  them ;  and  it  is,  therefore,  not 
uncommon  to  encounter  large  quantities  of  water  in  excavating 
tunnels  through  rock.  As  a  rule,  water  will  be  found  under 
high  mountains,  which  comes  from  the  melted  ice  and  snow 
percolating  through  the  rock  crevices. 

Some  detrital  soils,  like  gravel  and  sand,  are  pervious,  and 


8  TUNNELING 

others,  like  clay  and  shale,  are  impervious.  Detrital  soils 
lying  above  clay  are  almost  certain  to  carry  water  just  above 
the  clay  stratum.  In  tunnel  work,  therefore,  when  the  exca- 
vation keeps  well  within  the  clay  stratum,  little  trouble  is 
likely  to  be  had  from  water ;  should,  however,  the  excavation 
cut  the  clay  surface  and  enter  the  pervious  material  above, 
water  is  quite  certain  to  be  encountered.  The  quantity  of 
water  encountered  in  any  case  depends  upon  the  presence  of 
high  mountains  near  by,  and  upon  other  circumstances  which 
will  attract  the  attention  of  the  engineer. 

A  knowledge  of  the  pressure  of  the  water  is  desirable. 
This  may  be  obtained  by  observing  closely  its  source  and  the 
character  of  the  strata  through  which  it  passes.  Water 
coming  to  the  excavation  through  rock  crevices  will  lose 
little  of  its  pressure  by  friction,  while  that  which  has  passed 
some  distance  through  sand  will  have  lost  a  great  deal  of  its 
pressure  by  friction.  Water  bearing  sand,  and,  in  fact,  any 
water  bearing  detrital  material,  has  its  fluidity  increased  by 
water  pressure ;  and  when  this  reaches  the  point  where  flow 
results,  trouble  ensues.  The  streams  of  water  met  in  the 
construction  of  the  St.  Gothard  tunnel  had  sufficient  pressure 
to  carry  away  timber  and  materials. 


DETERMINING    THE    CENT  Ell    LINE 


CHAPTER  II. 

METHODS     OF     DETERMINING     THE      CENTER 

LINE   AND   FORMS   AND    DIMENSIONS   OF 

CROSS-SECTION. 


DETERMINING    THE    CENTER   LINE. 

TUNNELS  may  be  either  curvilinear  or  rectilinear,  but  the 
latter  form  is  the  more  common.  In  either  case  the  first  task 
of  the  engineer,  after  the  ends  of  the  tunnel  have  been  definitely 
fixed,  is  to  locate  the  center  line  exactly.  This  is  done  on  the 
surface  of  the  ground;  and  its  purpose  is  to  find  the  exact 
length  of  the  tunnel,  and  to  furnish  a  reference  line  by  which 
the  excavation  is  directed. 

Rectilinear  Tunnels.  —  In  short  tunnels  the  center  line  may  be 
accurately  enough  located  for  all  practical  purposes  by  means 
of  a  common  theodolite.  The  work  is  performed  on  a  calm, 
cleat  day,  so  as  to  have  the  instrument  and  observations  sub- 
jected to  as  little  atmospheric  disturbance  as  possible.  Wooden 
stakes  are  employed  to  mark  the  various  located  points  of  the 
center  line  temporarily.  The  observations  are  usually  repeated 
once  at  least  to  check  the  errors,  and  the  stakes  are  altered  as 
the  corrections  dictate  ;  and  after  the  line  is  finally  decided  to 
be  correctly  fixed,  they  are  replaced  by  permanent  monu- 
ments of  stone  accurately  marked.  The  method  of  checking  the 
observations  is  described  by  Mr.  \V.  D.  Haskoll  *  as  follows : 

44  Let  the  theodolite  be  carefully  set  up  over  one  of  the  stakes,  with  the 
nail  driven  into  it,  selecting  one  that  will  command  the  best  position  so  as  to 
range  backwards  and  forwards  over  the  whole  length  of  line,  and  also  obtain  a 
view  of  the  two  distant  points  that  range  with  the  center  line  ;  this  being  done, 

*  "  Prnotfrni  Tunneling."  %y  F.  "W.  Simnis. 


10  TUNNELING 

let  the  centers  of  every  stake  ...  be  carefully  verified.  If  this  be  carefully 
done,  and  the  centers  be  found  correct,  and  thoroughly  in  one  visual  1'r.e  as 
seen  through  the  telescope,  there  will  be  no  fear  but  that  a  perfectly  straight 
line  has  been  obtained. 

The  center  line  which  has  thus  been  located  on  the  ground 
surface  has  to  be  transposed  to  the  inside  of  the  tunnel  to 
direct  the  excavation.  To  do  this  let  A  and  B  be  the  entrances 
and  a  and  b  be  the  two  distinct  fixed  points  which  have  been 
ranged  in  with  the  center  line  located  on  the  ground  surface 
over  the  hill  Af  B,  Fig.  1.  The  instrument  is  set  up  at  V, 
any  point  on  the  line  A  a  produced,  and  a  bearing  secured  by 
observation  on  the  center  line  marked  on  the  surface.  This 
bearing  is  then  carried  into  the  tunnel  by  plunging  the  tele- 
scope, and  setting  pegs  in  the  roof  of  the  heading.  Lamps 


"A  v 

FIG.  1.  — Diagram  Showing  Manner  of  Lining  in  Rectilinear  Tunnels. 

hung  from  these  pegs  furnish  the  necessary  sighting  points. 
This  same  operation  is  repeated  on  the '  opposite  side  of  the 
hill  to  direct  the  excavation  from  that  end  of  the  tunnel. 
These  operations  serve  to  locate  only  the  first  few  points  inside 
the  tunnel.  As  the  excavation  penetrates  farther  into  the  hill, 
it  becomes  impossible  to  continue  to  locate  the  line  from  the 
outside  point,  and  the  line  has  to  be  run  from  the  points 
marked  on  the  roof  of  the  heading.  Great  accuracy  is  required 
in  all  these  observations,  since  a  very  small  error  at  the  begin- 
ning becomes  greater  and  greater  as  the  excavation  advances. 

In  very  long  tunnels  excavated  under  high  mountains  more 
elaborate  methods  have  to  be  adopted  for  locating  the  center 
line.  The  theodolites  employed  must  be  of  large  size  ;  in  ran- 
ging the  center  line  of  the  St.  Gothard  tunnel,  the  theodolite 
used  had  an  object  glass  eight  inches  in  diameter.*  Instead  of 

*  See  also  Simplon  Tunnel,  Chapter  IX. 


DETERMINING    THE    CENTER    LINE  11 

the  ordinary  mounting  a  masonry  pedestal  with  a  perfectly 
level  top  is  employed  to  support  the  instrument  during  the 
observations.  The  location  is  made  by  means  of  triangulation. 
The  various  operations  must  be  performed  with  the  greatest 
accuracy,  and  repeated  several  times  in  such  a  way  as  to  reduce 
the  errors  to  a  minimum,  since  the  final  meeting  of  the  head- 
ings depends  upon  their  elimination. 

The  St.  Gothard  tunnel  furnishes  perhaps  the  best  illus- 
tration of  careful  work  in  locating  the  center  line  of  long  recti- 
linear tunnels  of  any  tunnel  ever  built.  The  length  of  this 
tunnel  is  9.25  miles,  and  the  height  of  the  mountain  above  it 
is  very  great.  The  center  line  was  located  by  triangulation  by 


.Stabbiefto 


FIG.  2.— Triangulation  System  for  Establishing  the  Center  Line  of  the  St.  Gothard  TunneL 

two  different  astronomers  using  different  sets  of  triangles,  and 
working  at  different  times.  The  set  or  system  of  triangles  used 
by  Dr.  Koppe,  one  of  the  observers,  is  shown  by  Fig.  2 ;  it  con- 
sists of  very  large  and  quite  small  triangles  combined,  the 
latter  being  required  because  the  entrances  both  at  Airolo  and 
Goeschenen  were  so  low  as  to  permit  only  of  a  short  sight 
being  taken.  The  apices  of  the  triangles  were  located  by  means- 
of  the  contour  maps  of  the  Swiss  Alpine  Club.  Each  angle 
was  read  ten  times,  the  instrument  was  collimated  four  times 
for  each  reading,  and  was  afterwards  turned  off  5°  or  10°  to 
avoid  errors  of  graduation.  The  average  of  the  errors  in  read- 
ing was  about  one  second  of  arc.  The  triangulation  was  compen- 


12 


TUNNELING 


Wire 


sated  according  to  the  method  of  least  squares.  The  probable 
£iror  in  the  fixed  direction  was  calculated  to  be  0.8"  of  arc  at 
Goeschenen  and  0.7"  of  arc  at  Airolo.  From  this  it  was 
assumed  that  the  probable  deviation  from  the  true  center  would 
be  about  two  inches  at  the  middle  of  the  tunnel,  but  when  the 
headings  finally  met  this  deviation  was  found  to  reach  eleven 
inches. 

Comparatively  few  tunnels  are  driven  by  working  from  the 
entrances  alone,  the  excavation  being  usually  prosecuted  at 
several  points  at  once  by  means  of  shafts.  In  these  cases,  in 

order  to  direct  the  excavation  cor- 
rectly, it  is  necessary  to  fix  the 
center  line  on  the  bottom  of  the 
shaft.  This  is  accomplished  in 
two  ways,  —  one  being  employed 
when  the  shaft  is  located  directly 
over  the  center  line,  and  the  other 
when  the  shaft  is  located  to  one 
side  of  the  center  line. 

When  the  shaft  is  located  on 
the  center  line  two  small  pillars 
are  placed  on  opposite  edges  of 
the  shaft  and  collimating  with  the 
center .  line  as  shown  by  Fig.  3. 
On  these  two  pillars  the  points' 

corresponding  to  the  center  line  are  correctly  marked,  and  con- 
nected by  a  wire  stretched  between  them.  To  this  wire  two 
plumb  bobs  are  fastened  as  far  apart  as  possible.  These  plumb 
bobs  mark  two  points  on  the  center  line  at  the  bottom  of  the 
shaft,  and  from  them  the  line  is  extended  into  the  headings  as 
the  work  advances.  Compass  readings  are  employed  to  check 
the  transit  lines  ranged  on  the  plumb  bobs.  Where  there  are 
rocks  containing  iron  ore  a  miner's  transit  should  be  employed 
for  making  the  compass  reading. 

When  the  shaft  is  placed  at  one   side  of   the   tunnel   the 


FlG.  3.  —  Method  of  Transferring  the 
Center  Line  down  Center  Shafts. 


DETERMINING    THE    CENTER    LINE  IS 

pillars  or  bench  marks  are  placed  normal  to  the  center  line  on. 
the  edges  of  the  shaft  as  shown  by  Fig.  4.     Between  the  points 
A  and  B  a  wire  is  stretched,  and  from  it  two  plumb  bobs  are 
suspended,  as  described  in  the 
preceding  case  ;    these   plumb  ?A 

bobs  establish  a  vertical  plane 
normal  to  the  axis  of  the  tun- 
nel. The  excavation  of  the 
side  tunnel  is  carried  along  the 
line,  BW  until  it  intersects  the  -z 

line  of  the  main  tunnel,  whose 

center  line    is  determined   by        ~'Cefiter~  '  -0  "~Ifn~' 
measuring  off  underground  a  ~]W~ 

distance  equal   tO    the    distance    FIG.  4 -Method  of  Transferring  the  Center 

Line  down  Side  Shafts. 

B  0  on  the  surface.    By  setting 

the  instrument  over  the  under-ground  point  0,  and  turning  off 
a  right  angle  from  the  line  B  0,  the  center  line  of  the  tunnel  is 
extended  into  the  headings. 

Curvilinear  Tunnels.  —  There  are  various  methods  of  locating 

the  center  line  of  curvilinear  tunnels,  but  the  method  of  tangent 

offsets  is  the  one  most  commonly  employed.    It 

•p 

consists  in  finding  the  length  of  an  ordinate 
DC,  Fig.  5,  perpendicular  to  the  tangent  AX, 
at  a  point  D  taken  at  a  known  distance  AD 
=  d  from  the  point  of  tangent  A,  0  being  the 
center  of  the  arc  AB  and  OA  being  the  radius. 


F 


FIG.  5.  -  Diagram  From  0  draw  OZ  parallel  to  the  tangent  AX, 
of^De^^ng  and  Produce  the  perpendicular  DC  until  it  in- 
Tangent  offsets  tersects  the  line  OZ  at  E.  Join  0  and  (7. 
From  the  right-angle  triangle  OCE, 

OE  ==  AD  =  d 

CO  =  r  

EC=  V^^^   ...........     (1) 

ED  =  OA  =  r 

DC  =  r  -  EC  =  y  .     .     ....     .     (2) 


14 


TUNNELING 


TIG.  C.  —  Diagram  Showing  Method 
of  Determining  Tangent  Off  sets 
for  Arcs  of  over  90°. 


Substituting  these    values   in   equations    (1)    and   (2)   we 

have,  y  =  r  Vr2  —  d2. 

When  the  arc  AB  is  greater  than  a  quadrant,  as  in  Fig.  6, 

the  projection  AF  of  the  arc  becomes  equal  to  its  radius,  and 

for  any  value  of  d  between  this  pro- 
jection and  that  of  the  chord  AK 
there  are  thus  two  values  of  y,  viz., 
?/!  and  «/2,  both  deduced  from  the 
formula,  y  =  r  ±  Vr2  —  cP. 

Assuming  the  value  of  d  —  A  G-, 
to  locate  the  point  H,  GrN  =  yv  —  r 
—  Vr2  —  (P,  and  to  locate  the  point 
J,  <H=yi=r+*JJ*=ff. 
By  giving  to  d  the  values  between    0  and  AF,  the  various 

Tallies  for  the  tangent  offsets  are  obtained. 

In  staking  out  the  center  line  of  a  curvilinear  tunnel  the 

greatest  accuracy  is  required,  since  a  very  small  error  will  throw 

the  work  out  and  cause 

serious  trouble.    At  the 

beginning    the    excava- 
tion   is    conducted     as 

closely  as  may  be  to  the 

line   of  the   curve,  and 

as  soon  as  it  has  pro- 
gressed far  enough  the 

tangent  AT,  Fig.  7,  is 

ranged   out.      At   B   a 

point    is    located    over 

which  to  set  the  instru-   ( 

ment,  and  the  distance 

AB  is  measured  for  the 

purpose  of  finding  the  ordinate  of  the  right  angle  triangle  OAB. 

Now   OA  =  r,  AB  =  d,  and  <£  =  angle  ABO.      Then:  Tang. 


FIG.  7.  — Method  of  Laying  Out  the  Center  Line  of 
Curvilinear  Tunnels. 


DETERMINING    THE   CENTER    LINE  15 

Doubling  the  value  of  and  making  the  angle  ABC  =  2  <£, 
the  line  BC  \\i\l  be  fixed  and  the  point  C  located  by  taking 
AB  =  BC.  On  B  C  the  ordinates  are  laid  off  to  locate  the  curve. 
Prolong  CB  so  that  CD  =  CB.  Then  the  portion  of  the*  curve 
CE  is  symmetrical  with  CE,  and  the  ordinates  used  to  locate 
EC  may  be  employed  to  locate  (7F,  by  laying  them  off  in  the 
reverse  order. 

FORM    AND    DIMENSIONS    OF   CROSS-SECTION. 

In  deciding  upon  the  sectional  profile  of  a  tunnel  two  factors 
have  to  be  taken  into  consideration :  (1)  The  form  of  section 
best  suited  to  the  conditions,  and  (2)  the  interior  dimensions  of 
this  section. 

Form  of  Section.  —  The  form  of  the  sectional  profile  of  a  tun- 
nel should  be  such  that  the  lining  is  of  the  best  form  to  resist 
the  pressures  exerted  by  the  unsupported  walls  of  the  tunnel 
excavation,  and  these  vary  with  the  character  of  the  material 
penetrated.  These  pressures  are  both  vertical  and  lateral  in 
direction  ;  the  roof,  deprived  of  support  by  the  excavation,  tends 
to  fall,  and  the  opposite  sides  for  the  same  reason  tend  to  slide 
inward  along  a  plane  more  or  less  inclined,  depending  upon  the 
friction  and  cohesion  of  the  material.  In  some  rocks  the  co- 
hesion is  so  great  that  they  will  stand  vertically,  while  it  may 
be  very  small  in  loose  earth  which  slides  along  a  plane  whose 
inclination  is  directly  proportional  to  the  cohesion. 

From  the  theory  of  resistance  of  profiles  we  know  that  the 
resistance  of  a  line  to  exterior  normal  forces  is  directly  propor- 
tional to  its  degree  of  curvature,  and  consequently  inversely 
proportional  to  the  radius  of  the  curve.  Hence  the  sectional 
profile  of  a  tunnel  excavated  through  hard  rock,  where  there 
are  no  lateral  pressures  owing  to  the  great  cohesion  of  the  ma- 
terial, and  having  to  resist  only  the  vertical  pressure,  should 
be  designed  to  offer  the  greatest  resistance  at  its  highest  point, 
and  the  curve  must,  therefore,  be  sharper  there,  and  may  de- 
crease toward  the  base.  In  quicksand,  mud,  or  other  material 


16  TUNNELING 

practically  without  cohesion,  the  pressures  will  all  be  normal 
to  the  line  of  the  profile,  and  a  circular  section  is  the  one  best 
suited  to  resist  them.  These  theoretical  considerations  have 
been  proved  correct  by  actual  experience,  and  they  may  be 
employed  to  determine  in  a  general  way  the  form  of  section  to 
be  adopted.  Applying  them  to  very  hard  rock,  they  give  us 
a  section  with  an  arched  roof  and  vertical  side  walls.  In  softer 
materials  they  give  us  an  elliptical  section  with  its  major  axis 
vertical,  and  in  very  soft  quicksands  and  mud  they  give  us  the 
circular  section.  These  three  forms  of  cross-section  and  their 
modifications  are  the  ones  commonly  employed  for  tunnels. 
An  important  exception  to  this  general  practice,  however,  is 
met  with  in  some  of  the  underground  city  rapid-transit  rail- 
ways built  of  late  years,  where  a  rectangular  or  box  section  is 
employed.  These  tunnels  are  usually  of  small  depth,  so  that 
the  vertical  pressures  are  comparatively  light,  and  the  bending 
strains,  which  they  exert  upon  the  flat  roof,  are  provided  for  by 
employing  steel  girders  to  form  the  roof  lining. 

From  what  has  been  said  it  will  be  seen  that  it  is  impossible 
to  establish  a  standard  sectional  profile  to  suit  all  conditions. 
The  best  one  for  the  majority  of  conditions,  and  the  one  most 
commonly  employed,  is  a  polycentric  figure  in  which  the  num- 
ber of  centers  and   the 
length  of  the  radii  are 
fixed  by  the  engineer  to 
meet  the  particular  con- 
ditions which  exist.     In 
a  general  way  this  form 
f  of  center   may  be   con- 
sidered as  composed  of 
two    parts    symmetrical 
in  respect  to  the  vertical 

FIG.  8.  — Diagram  of  Polycentric  Sectional  Profile. 

axis.    Fig.  8  shows  such 

a  profile,  in  which  DH  is  the  vertical  axis.  The  section  is 
unsymmetrical  in  respect  to  the  horizontal  axis  GE.  The 


DETERMINING    THE    CENTER    LINE  17 

upper  part  forming  the  roof  arch  is  usually  a  serai-circle  or 
semi-oval,  while  the  lower  part,  comprising  the  side  walls 
and  invert  or  floor,  varies  greatly  in  outline.  Sometimes  the 
side  walls  are  vertical  and  the  invert  is  omitted,  as  shown  by 
Fig.  9 ;  and  sometimes  the  side  walls  are  inclined,  with  their 
bottoms  braced  apart  by  the  invert,  as  shown  by  Fig.  10.  In 
more  treacherous  soils  the  side  walls  are  curved,  and  are  con- 
nected by  small  curved  sections  to  the  invert,  as  shown  by  Fig. 


FI9-9  Fig.  10.  Fig.tl. 

FIGS   9  to  11.  — Typical  Sectional  Profiles  for  Tunnel. 

11.  In  the  last  example  the  side  walls  are  commonly  called 
skewbacks,  and  the  lower  part  of  the  section  is  a  polycentric 
figure  like  the  upper  part,  but  dissimilar  in  form. 

In  a  tunnel  section  whose  profile  is  composed  entirely  of 
arcs  the  following  conditions  are  essential:  The  centers  of  the 
springer  arcs  G-a  and  Ea! ',  Fig.  8,  must  be  located  on  the  line 
(jrE;  the  center  of  the  roof  arc  bDb'  must  be  located  on  the 
axis  HD ;  the  total  number  of  centers  must  be  an  odd  number ; 
the  radii  of  the  succeeding  arcs  from  Cr  toward  D  and  E  toward 
D  must  decrease  in  length,  and  finally  the  sum  of  the  angles 
subtended  by  the  several  arcs  must  equal  180°. 

Dimensions  of  Section.  —  The  dimensions  to  be  given  to  the 
cross-section  of  a  tunnel  depend  upon  the  purpose  for  which  it 
is  to  be  used.  Whatever  the  purpose  of  the  tunnel,  the  follow- 
ing three  points  have  to  be  considered  in  determining  the  size 
of  its  cross-section:  (1)  The  size  of  clear  opening  required;  (2) 
the  thickness  of  lining  masonry  necessary ;  and  (3)  the  decrease 
in  the  clear  opening  from  the  deformation  of  the  lining. 

Railway  tunnels  may  be  built  either  to  accommodate  one  or 


18 


TUNNELING 


two  tracks.  In  single-track  tunnels  a  clear  space  of  at  least  2£ 
ft.  on  each  side  should  be  allowed  for  between  the  tunnel  wall 
and  the  side  of  the  largest  standard  locomotive  or  car,  and  a 
clear  space  of  at  least  3  ft.  should  be  allowed  for  between  the 
roof  and  the  top  of  the  same  locomotive  or  car.  Since  the  roof 
of  the  tunnel  is  arch-shaped,  to  secure  a  clearance  of  3  ft.  at 
every  point  will  necessitate  making  the  clearance  at  the  center 
greater  than  this  amount.  In  double-track  tunnels  the  same 
amounts  of  side  and  roof  clearances  have  to  be  provided  for, 
and,  in  addition,  there  has  to  be  a  clearance  of  at  least  2  ft. 
between  trains  passing  on  the  two  tracks.  Referring  to  Fig.  8, 
and  assuming  the  line  AB  to  represent  the  level  of  the  tracks, 
then  the  ordinary  dimensions  in  feet  required  for  both  single- 
and  double-track  tunnels  are  as  follows  :  — 


HEIOHT,  D.  F. 
FEET. 

WIDTH,  G.  E. 
FEET. 

HEIO.HT,C.  F. 
FEET. 

HBIO.HT.C.H. 
FEET. 

Single  track 
Double  track    .     .     . 

17.6  to  18 
26.6  to  28 

16.5  to  18 
26.6  to  28 

6     to  7.4 
6..']  to  6.9 

1  to  \  AB 

i  to  4  A  B 

The  thickness  of  the  masonry  lining  to  be  allowed  for  varies 
with  the  material  penetrated,  as  will  be  explained  in  a  succeed- 
ing chapter  where  the  dimensions  for  various  ordinary  condi- 
tions are  given  in  tabular  form.  The  lining  masonry  is  subject 
to  deformation  in  three  ways :  by  the  sinking  of  the  whole 
masonry  structure,  by  the  squeezing  together  of  the  side  walls 
by  the  lateral  pressures,  and  by  the  settling  of  the  roof-arch. 
The  whole  masonry  structure  never  sinks  more  than  three  or 
four  inches,  and  merits  little  attention.  The  movement  of  the 
side  walls  towards  each  other,  which  may  amount  to  three  or 
four  inches  for  each  wall  without  endangering  their  stability, 
has,  however,  to  be  allowed  for ;  and  similar  allowance  must  be 
made  for  the  settling  of  the  roof-arch,  which  may  amount  to 
from  nine  inches  to  two  feet. 


EXCAVATING   MACHINES   AND   HOCK  DIULLS 


19 


CHAPTER   III. 

EXCAVATING    MACHINES    AND    ROCK     DRILLS: 
EXPLOSIVES    AND    BLASTING. 


Earth-Excavating  Machines. —  Comparatively  few  of  the  labor- 
saving  machines  employed  for  breaking  up  and  removing  loose 
soil  in  ordinary  surface  excavation  are  used  in  tunnel  excava- 
tion through  the  same  material.  Several  forms  of  tunnel 
excavating  machines  have  been  tried  at  various  times,  but  only 
a  few  of  them  have  attained  any  measure  of  success,  and  these 
have  seldom  been  employed  in  more  than  a  single  work.  In 
the  Central  London  underground  railway  work  through  clay  a 
continuous  bucket  excavator  (Fig.  12)  was  employed  with 


FIG.  12.— Soft  Ground  Bucket  Excavating  Machine  :  Central  London  Underground  Railway. 

considerable  saving  in  time  and  labor  over  hand  work,  and  in 
some  recent  tunnel  work  in  America  the  contractors  made 
quite  successful  use  of  a  modified  form  of  steam  shovel.  These 
are  the  most  recent  attempts  to  use  excavating  machines  in 
soft  ground,  and  they,  like  all  previous  attempts,  must  be 
classed  as  experiments  rather  than  as  examples  of  common 
practice.  The  shovel,  the  spade,  and  the  pick,  wielded  by 


20  TUNNELING 

hand,  are  the  standard  tools  now,  as  in  the  past,  for  excavating* 
soft-ground  tunnels. 

Rock-Excavating  Machines,  —  At  one  period  during  the  work 
of  constructing  the  Hoosac  tunnel  considerable  attention  was 
devoted  to  the  development  of  a  rock  excavating,  boring,  or 
tunneling  machine.  This  device  was  designed  to  cut  a  groove 
around  the  circumference  of  the  tunnel  thirteen  inches  wide 
and  twenty-four  feet  in  diameter  by  means  of  revolving  cutters. 
It  proved  a  failure,  as  did  one  of  smaller  size,  eight  feet  diame- 
ter, tried  subsequently.  During  and  before  the  Hoosac  tunnel 
work  a  number  of  boring-machines  of  similar  character  were 
experimented  with  at  the  Mont  Cenis  oinnel  and  elsewhere  in 
Europe ;  but,  like  the  American  devices,  they  were  finally 
abandoned  as  impracticable. 

Hand  Drills.  —  Briefly  described,  a  drill  is  a  bar  of  steel 
having  a  chisel-shaped  end  or  cutting-edge.  The  simplest  form 
of  hand  drill  is  worked  by  one  man,  who  holds  the  drill  in  one 
hand,  and  drives  it  with  a  hammer  wielded  by  his  other  hand. 
A  more  efficient  method  of  hand-drill  work  is,  however,  where 
one  man  holds  the  drill,  and  another  swings  the  hammer  or 
sledge.  Another  form  of  hand  drill,  called  a  churn  drill,  con- 
sists of  a  long,  heavy  bar  of  steel,  which  is  alternately  rained 
and  dropped  by  the  workman,  thus  cutting  a  hole  by  repeated 
impacts. 

In  drilling  by  hand  the  workman  holding  the  drill  gives  it  a 
partial  turn  on  its  axis  at  every  stroke  in  order  to  prevent 
wedging  and  to  offer  a  fresh  surface  to  the  cutting-edge.  For 
the  same  reason  the  chips  and  dust  which  accumulate  in  the 
drill-hole  are  frequently  removed.  The  instruments  used  for 
this  purpose  are  called  scrapers  or  dippers,  and  are  usually  very 
simple  in  construction.  A  common  form  is  a  strong  wire  hav- 
ing its  end  bent  at  right  angles,  and  flattened  so  as  to  make  a 
sort  of  scoop  by  which  the  drillings  may  be  scraped  or  hoisted 
out  of  the  hole.  It  is  generally  advantageous  to  pour  water 
into  the  drill-hole  while  drilling  to  keep  the  drill  from  heating. 


EXCAVATING    MACHINES   AND    HOCK   DRILLS  21 

Power  Drills.  —  When  the  conditions  are  such  that  use  can 
be  made  of  them,  it  is  nearly  always  preferable  to  use  power 
drills,  on  account  of  their  greater  speed  of  penetration  and 
greater  economy  of  work.  Power  drills  are  worked  by  direct 
steam  pressure,  or  by  compressed  air  generated  by  steam  or 
water  power,  and  stored  in  receivers  from  which  it  is  led  to  the 
drills  through  iron  pipes.  A  great  variety  of  forms  of  power 
drills  are  available  for  tunnel  work  in  rock,  but  they  can  nearly 
all  be  grouped  in  one  of  two  classes :  (1)  Percussion  drills,  and 
(2)  Rotary  drills. 

Percussion  Drills.  —  The  first  American  percussion  drill 
was  patented  by  Mr.  J.  J.  Couch  of  Philadelphia,  Penn.,  in 
March,  1849.  In  May  of  the  same  year,  Mr.  Joseph  W.  Fowle, 
who  had  assisted  Mr.  Couch  in  developing  his  drill,  patented  a 
percussion  drill  of  his  own  invention.  The  Fowle  drill  was 
taken  up  and  improved  by  Mr.  Charles  Burleigh,  and  was  first 
used  on  the  Hoosac  tunnel.  In  Europe  Mr.  Cave  patented 
a  percussion  drill  in  France  in  October,  1851.  This  invention 
was  soon  followed  by  several  others ;  but  it  was  not  until  Som- 
meiller's  drill,  patented  in  1857  and  perfected  in  1861,  was  used 
on  the  Mont  Cenis  tunnel,  that  the  problem  of  the  percussion 
drill  was  practically  solved  abroad.  Since  this  time  numer- 
ous percussion  drill  patents  have  been  taken  out  in  both 
America  and  Europe. 

A  percussion  drill  consists  of  a  cylinder,  in  which  works  a 
piston  carrying  a  long  piston  rod,  and  which  is  supported  in 
such  a  manner  that  the  drill  clamped  to  the  end  of  the  piston 
rod  alternately  strikes  and  is  withdrawn  from  the  rock  as  the 
piston  reciprocates  back  and  forth  in  the  cylinder.  Means  are 
devised  by  which  the  piston  rod  and  drill  turn  slightly  on  their 
axis  after  each  stroke,  and  also  by  which  the  drill  is  fed  for- 
ward or  advanced  as  the  depth  of  the  drill-hole  increases. 
The  drills  of  this  type  which  are  in  most  common  use  in 
America  are  the  Ingersoll-Sergeant  and  the  Rand.  There  are 
various  other  makes  in  common  use,  however,  which  differ 


22  TUNNELING 

from  the  two  named  and  from  each  other  chiefly  in  the  methods 
by  which  the  valve  is  operated.  All  of  these  drills  work  either 
with  direct  steam  pressure  or  with  compressed  air.  Workable 
percussion  drills  operated  by  electricity  are  built,  but  so  far 
they  do  not  seem  to  have  been  able  to  compete  commercially 
with  the  older  forms.  No  attempt  will  be  made  here  to  make 
a  selection  between  the  various  forms  of  percussion  drills  for 
tunnel  work,  and  for  the  differences  in  construction  and  the 
merits  claimed  for  each  the  reader  is  referred  to  the  makers  of 
these  machines.  All  of  the  leading  makes  will  give  efficient 
service.  It  goes  almost  without  saying  that  a  good  percussion 
drill  should  operate  with  little  waste  of  pressure,  and  should 
be  composed  of  but  few  parts,  which  can  be  easily  removed  and 
changed. 

Drill  Mountings.  - —  For  tunnel  work  the  general  European 
practice  is  to  mount  power  drills  upon  a  carriage  moving  on 
tracks  in  order  that  they  may  be  easily  withdrawn  during 
the  firing  of  blasts.  Connection  is  made  with  the  steam  or 
compressed  air  pipes  by  means  of  flexible  hose  which  can 
easily  be  attached  or  detached  as  the  drill  advances  or  when  it 
is  moved  for  repairs  or  during  blasts.  Two,  four,  and  sometimes 
more  drills  are  mounted  and  work  simultaneously  on  a  single 
carriage.  In  America  it  has  been  found  that  column  mount- 
ings have  been  more  successful  for  tunnel  work  than  any  other 
form.  The  column  mounting  made  by  the  Ingersoll-Sergeant 
Drill  Co.  is  shown  by  Fig.  13.  In  using  this  form  of  mounting 
no  tracks  or  other  special  apparatus  is  required ;  it  is  not 
necessary,  as  is  the  case  with  the  carriage  mounting,  to  remove 
the  debris  before  resuming  operations,  but  as  soon  as  the  blast- 
ing has  been  finished  and  £he  smoke  has  sufficiently  disap- 
peared the  column  can  be  set  up  and  drilling  resumed. 

Rotary  Drills.  —  Rotary  drilling  machines,  or  more  simply 
rotary  drills,  were  first  used  in  1857  in  the  Mont  Cenis  tunnel. 
The  advantages  claimed  for  rotary  drills  in  comparison  with 
percussion  drills  are :  (1)  That  less  power  is  required  to  drive 


EXCAVATING    MACHINES    AND    HOCK   DRILLS  23 

the  drill,  and  the  power  is  better  utilized ;  (2)  once  the  ma- 
chines work  easily  they  do  not  require  continual  repairs,  and 
(3)  in  driving  holes  of  large  size  the  interior  nucleus  is  taken 


FlG.  13.  — Column  Mounting  for  Percussion  Drill :  Ingersoll-Sergeant  Drill  Co. 

away  intact,  thus  reducing  work  and  increasing  the  speed  of 
drilling.  Rotary  drills  are  extensively  used  for  geological, 
mining,  well-driving,  and  prospecting  purposes;  but  they  are 
very  seldom  employed  in  tunnels  in  America,  although  success 


24  TUNNELING 

fully  used  for  this  purpose  in  Europe.  The  reason  they  have 
not  gained  more  favor  among  American  tunnel  builders  is  due  to 
some  extent  perhaps  to  prejudice,  but  chiefly  to  the  great  cost 
of  the  machine  as  compared  with  percussion  drills,  and  to  the 
expense  of  diamonds  for  repairs.  Those  who  advocate  these 
machines  for  tunnel  work  point  out,  however,  that  under  ordi- 
nary usage  the  diamonds  have  a  very  long  life,  —  borings  of 
700  lin.  ft.  being  recorded  without  repairs  to  the  diamonds. 

The  form  of  rotary  drill  used  chiefly  for  prospecting  pur- 
poses is  the  diamond  drill.  This  machine  consists  of  a  hollow 
cylindrical  bit  having  a  cutting-edge  of  diamonds,  which  is 
revolved  at  the  rate  of  from  two.  hundred  to  four  hundred 
revolutions  per  minute  by  suitable  machinery  operated  by  steam 
or  compressed  air.  The  diamonds  are  set  in  the  cutting-edge  of 
the  bit  so  as  to  project  outward  from  its  annular  face  and  also 
slightly  inside  and  outside  of  its  cylindrical  sides  (Fig.  14). 
When  the  drill  rod  with  the  bit  at- 
tached is  rotated  and  fed  forward  the 
bit  cuts  an  annular  hole  into  the  rock ; 
the  drillings  being  removed  from  the 
hole  by  a  constant  stream  of  water 
which  is  forced  down  through  the  hol- 

FIG.  14.  —  Sketch  of  Diamond        i          j   MI        'j         j  ±1 

Drill  Bit.  *ow  drill  roc*  an(*  emerges,  carrying  the 

debris  with  it,  up  through  the  narrow 

space  between  the  outside  of  the  bit  and  the  walls  of  the  hole. 
There  are  various  makes  of  diamond  drills,  but  they  all  operate 
in  essentially  the  same  manner. 

The  rotary  drill  principally  employed  in  Europe  in  tunneling 
is  the  Brandt.  The  cutting-edge  of  the  Brandt  drill  consists  of 
hardened  steel  teeth.  The  bit  is  pressed  against  the  rock  by 
hydraulic  pressure,  and  usually  makes  from  seven  to  eight  revo- 
lutions per  minute.  Some  of  the  water  when  freed  goes 
through  the  hollow  bit,  keeping  it  cool,  and  cleaning  the  hole  of 
debris.  A  water  pressure  of  from  300  to  450  Ibs.  per  square 
inch  is  required  to  operate  these  drills.  Rotary  rock-drills 


EXCAVATING   MACHINES   AND   HOCK   DULLLS  25 

may  be  mounted  either  on  carriages  or  on  columns  for  tunnel 
work. 

EXPLOSIVES  AND  BLASTING. 

^ 

When  the  holes  are  once  drilled,  either  by  hand  or  power 
drills,  they  are  charged  with  explosives.  The  principal  explo- 
sives employed  in  tunneling  are  gunpowder,  nitroglycerine,  and 
dynamite. 

Gunpowder Gunpowder  is  composed  of  charcoal,  sulphur, 

and  saltpeter  in  proportions  varying  according  to  the  quality  of 
the  powder.  For  mining  purposes  the  composition  employed 
is  65  %  saltpeter,  15  <jc  sulphur,  and  20  c/c  charcoal.  It  is  a  black 
granulated  powder  having  a  specific  gravity  of  1.5 ;  the  black 
color  is  given  by  the  charcoal ;  and  the  grains  have  an  angular 
form,  and  vary  in  size  from  1  in.  to  £  in.  Good  blasting 
powder  should  contain  no  fine  grains,  which  may  be  detected 
by  pouring  some  of  the  powder  upon  a  sheet  of  white  paper. 
The  force  developed  by  the  explosion  of  gunpowder  is  not 
accurately  known ;  it  depends  upon  the  space  in  which  it  is 
confined.  Different  authorities  estimate  the  pressure  at  from 
15,000  Ibs.  per  sq.  in.  in  loose  blasts  to  200,000  Ibs.  per  sq.  in. 
in  gunnery.  Authorities  also  differ  in  opinion  as  to  the 
character  of  the  gases  developed  by  the  explosion  of  gun- 
powder, a  matter  of  vital  concern  to  the  tunnel  engineer,  since 
they  are  likely  to  affect  the  health  and  comfort  of  his  work- 
men. It  may  be  assumed  in  a  general  way,  however,  that  the 
oxygen  of  the  saltpeter  converts  nearly  all  of  the  carbon  of 
the  charcoal  into  carbon  dioxide,  a  portion  of  which  combines 
with  the  potash  of  the  saltpeter  to  form  carbonate  of  potash, 
the  remainder  continuing  in  the  form  of  gas.  The  sulphur  is 
converted  into  sulphuric  acid,  and  forms  a  sulphate  of  potash, 
which  by  reaction  is  decomposed  into  hyposulphite  and  sul- 
phide. The  nitrogen  of  the  saltpeter  is  almost  entirely  evolved 
in  a  free  state ;  and  the  carbon  not  having  been  wholly  burnt 
into  carbonic  acid,  there  is  a  proportion  of  carbonic  oxide. 


26  TUNNELING 

Nitroglycerine.  —  Nitroglycerine  is  one  of  the  modern  explo- 
sives used  as  a  substitute  for  gunpowder.  It  is  a  fluid  pro- 
duced by  mixing  glycerine  with  nitric  and  sulphuric  acids ;  it 
freezes  at  +41°  F.,  and  burns  very  quietly,  developing  carbonic 
acid,  nitrogen,  oxygen,  and  Avater.  By  percussion  or  by  the 
explosion  of  some  substances,  such  as  capsules  of  gunpowder 
or  fulminate  of  mercury,  nitroglycerine  produces  a  sudden 
explosion  in  which  about  1,250  volumes  of  gases  are  pro- 
duced. The  pressure  of  these  gases  has  been  calculated  at 
26,000  atmospheres,  or  324,000  Ibs.  per  sq.  in.  Nitroglycerine 
explodes  very  easily  by  percussion  in  its  normal  state,  but  with 
great  difficulty  when  frozen ;  hence,  in  America,  at  the  begin- 
ning of  its  use,  it  was  transported  only  in  a  frozen  state.  When 
dirty,  nitroglycerine  undergoes  a  spontaneous  decomposition 
accompanied  by  the  development  of  gases  and  the  evolution 
of  heat,  which,  reaching  388°  F.,  causes  it  to  explode.  Not- 
withstanding the  enormous  pressures  which  nitroglycerine  de- 
velops, it  is  very  seldom  used  in  its  liquid  state,  but  is  mixed 
with  a  granular  absorbent  earth  composed  of  the  shells  of 
diatoms.  The  fluid  undergoes  no  chemical  change  by  being 
absorbed,  and  explodes,  freezes,  and  burns  under  the  same  con- 
ditions as  in  the  fluid  state. 

Dynamite.  —  The  credit  of  rendering  nitroglycerine  available 
for  the  purposes  of  the  engineer  by  mixing  it  with  a  granular 
absorbent  is  due  to  Albert  Nobel  of  Stockholm,  Sweden,  who 
named  the  new  material  dynamite.  The  nitroglycerine  in 
dynamite  loses  very  little  of  its  original  explosive  power,  but 
is  very  much  less  easily  exploded  by  percussion,  and  can  l>e 
employed  in  horizontal  as  well  as  vertical  holes,  which  was,  of 
course,  not  possible  in  its  liquid  state.  Dynamite  must  contain 
at  least  50  $  of  nitroglycerine.  Some  manufacturers,  instead  of 
diatomaceous  earth,  use  other  absorbents  which  develop  gases 
upon  explosion  and  increase  the  force  of  the  explosion.  These 
mixtures  are  classed  under  the  general  name  of  false  dyna- 
mites. A  great  many  varieties  of  dynamite  are  manufactured, 


EXCAVATING    MACHINES    AND    ROCK    DRILLS  27 

and  each  manufacturer  usually  makes  a  number  of  grades  to 
which  he  gives  special  names.  Dynamite  for  railway  work, 
tunneling,  and  mining  contains  about  50  %  of  nitroglycerine  ;  for 
quarrying  about  35  f/,  and  for  blasting  soft  rocks  aboufc  30  $. 
It  is  sold  in  cylindrical  cartridges  covered  with  paper. 

Storage  of  Explosives In  driving  tunnels  through  rock 

large  quantities  of  explosives  must  be  used,  and  it  is  necessary 
to  have  some  safe  place  for  storing  them.  In  many  States 
there  are  special  laws  governing  the  transportation  and  storage 
of  explosives ;  where  there  is  no  regulation  by  law  the  engineer 
should  take  suitable  precautions  of  his  own  devising.  It  is 
best  to  build  a  special  house  or  hut  in  one  of  the  most  con- 
cealed portions  of  the  work  and  away  from  the  tunnel,  and 
protect  it  with  a  lightning-rod  and  from  fire.  Strict  orders 
should  be  given  to  the  watchman  in  charge  not  to  allow  persons 
inside  with  lamps  or  fire  in  any  form,  and  smoking  should  be 
prohibited.  The  use  of  hammers  for  opening  the  boxes 
should  be  prohibited ;  and  dynamite,  gunpowder,  and  fulminate 
of  mercury  should  not  be  stored  together  in  the  same  room. 
A  quantity  of  dynamite  for  two  or  three  days'  consumption 
may  be  stored  near  the  entrance  of  the  tunnel  in  a  locked  box, 
the  keys  of  which  are  kept  by  the  foreman  of  the  work. 
When  dynamite  has  been  frozen  the  engineer  should  provide 
some  arrangement  by  which  it  may  be  heated  to  a  temperature 
not  exceeding  120°  F.,  and  absolutely  forbid  it  being  thawed 
out  on  a  stove  or  by  an  open  fire. 

Fuses When  gunpowder  is  used  in  tunneling  it  is  ignited 

by  the  Blickford  match.  This  match,  or  fuse  as  it  is  more 
commonly  called,  consists  of  a  small  rope  of  yarn  or  cotton 
having  as  a  core  a  small  continuous  thread  of  fine  gunpowder. 
To  protect  the  outside  of  the  fuse  from  moisture  it  is  coated 
with  tar  or  some  other  impervious  substance.  These  fuses  are 
so  well  made  that  they  burn  very  uniformly  at  the  rate  of 
about  1  ft.  in  20  seconds,  hence  the  moment  of  explosion  can 
be  pretty  accurately  fixed  beforehand.  Blickford  matches 


28  TUNNELING 

have  the  objection  for  tunnel  work  of  burning  with  a  bad  odor, 
especially  when  they  are  coated  with  tar,  and  to  remedy  this 
many  others  have  been  invented.  Those  of  Rzika  and  Franzl  are 
the  best  known  of  these.  The  former  has  many  advantages,  but 
it  burns  too  quickly,  about  3  ft.  per  second,  and  is  expensive ; 
the  latter  consists  of  a  small  hollow  rope  filled  with  dynamite. 

Blickford  matches  cannot  be  used  to  explode  dynamite,  the 
use  of  a  cartridge  being  required.  These  cartridges  are  small 
copper  cylinders  containing  fulminate  of  mercury.  They  may 
be  attached  to  the  end  of  the  Blickford  match,  which  being 
ignited  the  spark  travels  along  its  length  until  it  reaches  the 
copper  cylinder,  where  it  explodes  the  fulminate  of  mercury, 
which  in  turn  explodes  the  dynamite.  Blasts  may  also  be  fired 
by  electricity,  which,  in  fact,  is  the  most  common  and  the 
preferable  method,  because  several  blasts  can  be  fired  simulta- 
neously, and  because  the  current  is  turned  on  at  a  great  dis- 
tance, thus  affording  greater  safety  to  the  workmen. 

The  method  of  electric  firing  generally  employed  in  America 
is  known  as  the  connecting  series  method,  and  consists  in  firing- 
several  mines  simultaneously.  The  ends  of  the  wires  are 
scraped  bare,  and  the  wire  of  the  first  hole  of  the  series  is 
twisted  together  with  the  wire  of  the  second  hole,  and  so  on ; 
finally  the  two  odd  wires  of  the  first  and  last  holes  are  connected 
to  two  wires  of  a  single  cable  or  to  two  separate  cables  extend- 
ing to  some  safe  place  to  which  the  men  can  retreat.  Here  the 
two  cable  wires  are  connected  by  binding  screws  to  the  poles  of 
a  battery,  or  sometimes  to  a  frictional  electric  machine.  The  cur- 
rent passes  through  the  wires,  making  a  spark  at  each  break,  and 
so  fires  the  fulminate  of  mercury,  which  explodes  the  dynamite. 

Simultaneous  firing  by  electricity  by  utilizing  the  united 
strength  of  the  blasts  at  the  same  instant  secures  about  10$ 
greater  efficiency  from  the  explosives.  Another  advantage 
of  electric  firing  is  that  in  case  of  a  missfire  of  any  one  of  the 
holes  there  is  slight  possibility  of  explosion  afterwards,  and  the 
place  can  be  approached  at  once  to  discover  the  cause. 


EXCAVATING   MACHINES   AND    HOCK    DRILLS  29 

Tamping.  —  Tamping  is  the  material  placed  in  the  hole  above 
the  explosive  to  prevent  the  gases  of  explosion  from  escaping 
into  the  air.  Tamping  generally  consists  of  clay.  When  gun- 
powder is  used  the  clay  must  be  well  rammed  with  a  wooden 
tool,  and  paper,  cotton,  or  some  other  dry  material  must  be 
placed  between  the  moist  clay  and  the  powder.  When  dyna- 
mite is  used  it  is  not  necessary  to  ram  the  tamping,  since  the 
suddenness  of  the  explosion  shatters  the  rock  before  the  clay 
can  be  driven  from  the  hole. 

A  few  experienced  men  should  be  appointed  to  fire  the 
blasts.  These  men  should  give  ample  warning  previous  to  the 
blast  in  order  that  all  machinery  and  tools  which  might  be 
injured  by  flying  fragments  may  be  removed  out  of  danger,  and 
so  that  the  workmen  may  seek  safety.  When  all  is  ready  they 
should  fire  the  blasts,  keeping  accurate  count  of  the  explosions 
to  ensure  that  no  holes  have  missed  fire,  and  should  call  the 
workmen  back  when  all  danger  is  over.  In  case  any  hole  has 
missed  fire  it  should  be  marked  by  a  red  lamp  or  flag. 

Nature  of  Explosions.  —  When  the  explosives  are  ignited  a 
sudden  development  of  gases  results,  producing  a  sudden  and 
violent  increase  of  pressure,  usually  accompanied  by  a  loud 
report.  The  energy  of  the  explosion  is  exerted  in  all  directions 
in  the  form  of  a  sphere  having  its  center  at  the  point  of  explo- 
sion, and  the  waves  of  energy  lose  their  force  as  the  distance 
from  this  central  point  increases.  The  energy  of  the  explosion 
at  any  point  in  the  sphere  of  energy  is,  therefore,  inversely 
proportional  to  the  distance  of  this  point  from  the  center  of 
explosion.  In  the  vicinity  of  the  center  of  explosion  the  gases 
have  sufficient  power  to  destroy  the  force  of  cohesion  and 
shatter  the  rock ;  further  on,  as  they  lose  strength,  they  only 
destroy  the  elasticity  of  the  material  and  produce  cracks ;  and 
still  further  away  they  only  produce  a  shock,  and  do  not  affect 
the  material.  Within  the  sphere  of  energy  there  are,  therefore, 
three  other  concentric  spheres:  the  first  one  being  where 
cohesion  is  destroyed,  the  second  where  elasticity  is  overcome, 


30 

and  the  third  where  the  shock  is  transmitted  by  elasticity, 
When  the  latter  sphere  comes  below  the  surface,  the  gases 
remain  inside  the  rock;  but  when  the  surface  intersects  either 
of  the  other  two  spheres,  the  gases  blow  up  the  rock,  forming  a 
cone  or  crater,  whose  apex  is  at  the  point  of  explosion,  and 
which  is  called  the  blasting-cone.  The  larger  the  blasting-cone 
is,  the  greater  is  the  amount  of  rock  broken  up ;  and  the  object 
of  the  engineer  should,  therefore,  always  be  so  to  regulate  the 
depth  of  the  hole  and  the  quantity  of  explosive  as  to  secure  the 
largest  possible  blasting  cone  in  each  case.  Experiments  are 
required  to  determine  the  most  efficient  depth  of  hole,  and 
quantity  of  explosive  to  be  employed,  since  these  differ  in 
different  kinds  of  rock,  with  the  position  of  the  rock  strata, 
etc. ;  but  in  ordinary  practice,  the  depths  of  the  holes  are  made 
from  1^  ft.  to  2  ft.  in  the  heading  and  upper  portion  of  the 
tunnel,  when  drilled  by  hand;  and  from  3  ft.  to  5  ft.  when 
drilled  by  power  drills.  In  the  lower  portion  of  the  profile,  the 
holes  are  made  deeper,  from  3  ft.  to  4  ft.  when  drilled  by 
hand,  and  exceeding  5  ft.  when  drilled  by  power.  The  dis- 
tance of  the  holes  apart  should  be  about  equal  to  the  diameter 
of  the  blasting-cone ;  as  a  general  rule  it  is  assumed  that  the 
base  of  the  blasting-cone  has  a  diameter  equal  to  twice  the 
depth  of  the  hole.  The  following  table  gives  the  average 
number  of  holes  required  in  each  part  of  the  excavation  for  the 
St.  Gothard  tunnel : 

NO.  OF  PABT*  NAME    OF  PART  NO.  OF  HOLES 

1.  Heading 6  to  9 

2.  Right  wing  of  heading 3  to  5 

3.  Left  wing  of  heading 3  to  5 

4.  Shallow  trench  with  core 2 

5.  Deepening  of  trench  to  floor 6  to  9 

6.  Narrow  mass  of  core  to  left 3 

7.  Greater  mass  of  core  to  left 6  to  9 

8.  Culvert 1 

Total  section      .....      30  to  43 

*  The  location  of  the  parts  numbered  is  shown  by  Fig.  15,  p.  32. 


EXCAVATING    MACHINES    AND    ROCK    DRILLS  31 

The  quantity  of  explosives  required  for  blasting  depends 
upon  the  quality  of  the  rock,  since  the  force  of  the  explosives 
must  overcome  the  cohesion  of  the  rock,  winch  varies  with  its 
nature,  and  often  differs  greatly  in  rocks  of  the  same  kind  and 
composition.  The  quantity  of  explosives  required  to  secure 
the  greatest  efficiency  in  blasting  any  particular  rock  may  be 
determined  experimentally,  but  in  practice  it  is  usually  deduced 
by  the  following  rules :  (1)  The  blasting  force  is  directly  pro- 
portional to  the  weight  of  the  explosives  used,  and  (2)  the  bulk 
of  the  blasted  rock  is  proportional  to  the  cube  of  the  depth  of 
the  holes.  It  is  usually  assumed,  also,  that  the  explosive 
should  fill  at  least  one-fourth  the  depth  of  the  hole. 


32 


TUNNELING 


CHAPTER   IV. 

GENERAL  METHODS  OF  EXCAVATION:   SHAFTS: 
CLASSIFICATION    OF    TUNNELS. 


A  NUMBEII  of  different  modes  of  procedure  are  followed  in 
excavating  tunnel?,  and  each  of  the  more  important  of  these 
will  be  considered  in  a  separate  chapter.  There  are,  however, 
certain  characteristics  common  to  all  of  these  methods,  and 
these  will  be  noted  briefly  here. 

Division  of  Section.  —  It  may  be  asserted  at  the  outset  that 
the  whole  area  of  the  tunnel  section  is  not  ordinarily  excavated 
at  one  time,  but  that  it  is  removed  in  sections,  and  as  each 

section  is  excavated  it  is  thoroughly 
timbered  or  strutted.  The  order  in 
which  these  different  sections  are 
excavated  varies  with  the  method  of 
excavation,  and  it  is  clearly  shown 
for  each  method  in  succeeding  chap- 
ters. As  a  single  example  to  illus- 
trate the  proposition  just  made,  the 
division  of  the  section  and  the  se- 
quence  of  excavation  adopted  at  the 

0-^1,1         i,  t    •  i        .     J    /T^- 

^t.   (jrOthard   tuilliel   IS  Selected  (*lg. 

15).  The  different  parts  of  the 
section  were  excavated  in  the  order  numbered  ;  the  names  given 
to  each  part,  and  the  number  of  holes  employed  in  breaking  it 
down,  are  given  by  the  table  on  page  30.  Whatever  method  is 
employed,  the  work  always  begins  by  driving  a  heading,  which 
is  the  most  difficult  and  expensive  part  of  the  excavation.  All 
the  other  operations  required  in  breaking  down  the  remainder 


FIG.  15.—  Diagram  Showing  Sequence 
of   Excavation    lor  St.  Gothard 


CKNKKAL    METHODS    OF    EXCAVATION  33 

of  the  tunnel  section  are  usually  designated  by  the  general 
term  of  enlargement  of  the  profile.  The  various  operations  of 
excavation  may,  therefore,  be  classified  either  as  excavation 
of  the  heading  or  enlargement  of  the  profile. 

Excavation  of  the  Heading.  —  -  There  is  considerable  confusion 
among  the  different  authorities  regarding  the  exact  definition 
of  the  term  "heading"  as  it  is  used  in  tunnel  work.  Some 
authorities  call  a  small  passage  driven  at  the  top  of  the  profile 
a  heading,  and  a  similar  passage  driven  at  the  bottom  of  the 
profile  a  drift ;  others  call  any  passage  driven  parallel  to  the 
tunnel  axis,  whether  at  the  top  or  at  the  bottom  of  the  profile, 
a  drift;  and  still  others  give  the  name  "heading"  to  all  such 
passages.  For  the  sake  of  distinctness  of  terminology  it  seems 
preferable  to  call  the  passage  a  heading  when  it  is  located  at 
the  top  of  the  profile,  and  a  drift  when  it  is  located  near  the 
bottom. 

Headings  and  drifts  are  driven  in  advance  of  the  general 
excavation  for  the  following  purposes:  (1)  To  fix  correctly 
the  axis  of  the  tunnel;  (2)  to  allow  the  work  to  go  on  at 
different  points  without  the  gangs  of  laborers  interfering  with 
each  other ;  (3)  to  detect  the  nature  of  material  to  be  dealt  with 
and  to  l>e  ready  in  any  contingency  to  overcome  any  trouble 
caused  by  a  change  in  the  soil ;  and  (4)  to  collect  the  water. 
The  dimensions  of  headings  in  actual  practice  vary  according 
to  the  nature  of  the  soil  through  which  they  are  driven.  As 
a  general  rule  they  should  not  be  less  than  7  ft.  in  height,  so  as 
to  allow  the  men  to  work  standing,  and  have  room  left  for  the 
roof  strutting.  The  width  should  not  be  less  than  6  ft.,  to 
allow  two  men  to  work  at  the  front,  and  to  give  room  for 
the  material  cars  without  interfering  with  the  wall  strutting. 
Usually  headings  are  made  8  ft.  wide.  The  length  of  headings 
in  practice  varies  according  to  circumstances.  In  very  long 
tunnels  through  hard  rock  the  headings  are  sometimes  ex- 
cavated from  1000  ft;  to  2000  ft.  in  advance,  in  order  that  they 
may  meet  as  soon  as  possible  and  the  ranging  of  the  center  line 


34 


TUNNELING 


be  verified,  and  so  that  as  great  an  area  of  rock  as  possible  may 
be  attacked  at  the  same  time  in  the  work  of  enlarging  the 
profile.  In  short  tunnels,  where  the  ranging  of  the  center  line 
is  less  liable  to  error,  shorter  headings  are  employed,  and  in  soft 
soils  they  are  made  shorter  and  shorter  as  the  cohesion  of  the  soil 
decreases.  When  the  material  has  too  little  cohesion  to  stand 
alone,  the  tops  and  sides  of  the  heading  require  to  be  supported 
by  strutting.  To  prevent  caving  at  the  front  of  the  heading, 
the  face  of  the  excavation  is  made  inclined,  the  inclination 
following  as  near  as  may  be  the  natural  slope  of  the  material. 

Enlargement  of  the  Profile.  —  The  enlargement  of  the  profile 
is    accomplished    by    excavating    in    succession    several    small 

prisms  parallel  to  the  heading,  and 
its  full  length,  which  are  so  located 
that  as  each  one  is  taken  out  the 
cross-section  of  the  original  heading 
is  enlarged.  The  number,  location, 
and  sequence  of  these  prisms  vary 
in  different  methods  of  excavation, 
and  are  explained  in  succeeding 
chapters  where  these  methods  are 
described.  To  direct  the  excava- 

Fio.  16.  — Diagram  Showing  Manner 

of  Determining  Correspondence  of      tioil    SO    as    to   keep  it  always  Within 
Excavation  to  Sectional  Profile.  ,11  n  <»      , 

the  boundaries  of  the  adopted  pro- 
file, the  engineer  first  marks  the  center  line  on  the  roof  of  the 
heading  by  wooden  or  metal  pegs,  or  by  some  other  suitable 
means  by  which  a  plumb  line  may  be  suspended.  He  next 
draws  to  a  large  scale  a  profile  of  the  proposed  section ;  and 
beginning  at  the  top  of  the  vertical  axis  he  draws  horizontal 
lines  at  regular  intervals,  as  shown  by  Fig.  16,  until  they  inter- 
sect the  boundary  lines  of  the  profile,  and  designates  on  each 
of  these  lines  the  distance  between  the  vertical  axis  and  the 
point  where  it  intersects  the  profile.  It  is  evident  that  if  the 
foreman  of  excavation  divides  his  plumb  line  in  a  manner  corre- 
sponding to  the  engineer's  drawing,  and  then  measures  horizon- 


GENERAL  METHODS  OF  EXCAVATION 


35 


tally  and  at  right  angles  to  the  vertical  center  plane  of  the 
tunnel  the  distance  designated  on  the  horizontal  lines  of  the 
drawing,  he  will  have  located  points  on  the  profile  of  the  sec- 
tion, or  in  other  words  have  established  the  limits  of  excava- 
tion. 

In  the  excavation  of  the  Croton  Aqueduct  for  the  water 
supply  of  New  York  city,  an  instrument  called  a  polar  pro- 
tractor was  used  for  determining  the  location  of  the  sectional 


FIG.  17.  — Polar  Protractor  for  Determining  Profile  of  Excavated  Cross-Section. 

profile.  This  instrument  consists  of  a  circular  disk  graduated 
to  degrees,  and  mounted  on  a  tripod  in  such  a  manner  that 
it  may  be  leveled  up,  and  also  Lave  a  vertical  motion  and  a 
motion  about  the  vertical  axis.  The  construction  is  shown 
clearly  by  Fig.  17.  In  use  the  device  is  mounted  with  its 
center  at  the  axis  of  the  tunnel.  A  light  wooden  measuring- 
rod  tapering  to  a  point,  shod  with  brass  and  graduated  to  feet 
and  hundredths  of  a  foot,  lies  upon  the  wooden  arm  or  rest, 
which  revolves  upon  the  face  of  the  disk,  and  slides  out  to 


36  TUNNELING 

a  contact  with  the  surface  of  the  excavation  at  such  points 
as  are  to  be  determined.  If  the  only  information  desired  is 
whether  or  not  the  excavation  is  sufficient  or  beyond  the  es- 
tablished lines,  the  rod  is  set  to  the  proper  radius,  and  if  it 
swings  clear  the  fact  is  determined.  If  a  true  copy  of  the 
actual  cross-section  is  desired,  the  rod  is  brought  into  contact 
with  the  significant  points  in  the  cross-section,  and  the  angles 
and  distances  are  recorded. 

The  general  method  of  directing  the  excavation  in  enlarging 
the  profile  by  referring  all  points  of  the  profile  to  the  vertical 
axis  is  the  one  usually  employed  in  tunneling,  and  gives  good 
results.  It  is  considered  better  in  actual  practice  to  have  the 
excavation  exceed  the  profile  somewhat  than  to  have  it  fall 
short  of  it,  since  the  voids  can  be  more  easily  filled  in  with 
riprap  than  the  encroaching  rock  can  be  excavated  during  the 
building  of  the  masonry.  In  tunnels  where  strutting  is  neces- 
sary the  excavation  must  be  made  enough  larger  than  the 
finished  section  to  provide  the  space  for  it.  In  sofkground 
tunnels  it  is  also  usual  to  enlarge  the  excavation  to  allow  for 
the  probable  slight  sinking  of  the  masonry.  The  proper  allow- 
ance for  strutting  is  usually  left  to  the  judgment  of  the  fore- 
man of  excavation,  but  the  allowance  for  settlement  must  be 
fixed  by  the  engineer. 

SHAFTS. 

Shafts  are  vertical  walls  or  passages  sunk  along  the  line  of 
the  tunnel  at  one  or  more  points  between  the  entrances,  to 
permit  the  tunnel  excavation  to  be  attacked  at  several  different 
points  at  once,  thus  greatly  reducing  the  time  required  for 
excavation.  Shafts  may  be  located  directly  over  the  center 
of  the  tunnel  or  to  one  side  of  it,  and,  while  usually  vertical, 
are  sometimes  inclined.  During  the  construction  of  the  tunnel 
the  shafts  serve  the  same  purpose  as  the  entrances ;  hence  they 
must  afford  a  passageway  for  the  excavated  materials,  which 


GENERAL  METHODS  OF  EXCAVATION  37 

have  to  be  hoisted  out,  and  also  for  the  construction  tools  and 
materials  which  have  to  be  lowered  down  them.  They  must 
also  afford  a  passageway  for  workmen,  draft  animals,  and  for 
pipes  for  ventilation,  water,  compressed  air,  etc.  The  character 
of  this  traffic  indicates  the  dimensions  required,  but  these  de- 
pend also  upon  the  method  of  hoisting  employed.  Thus,  when 
a  windlass  or  horse  gin  is  used,  and  the  materials  are  hoisted 
in  buckets  of  small  dimensions,  the  dimensions  of  the  shaft  may 
also  be  small;  but  when  steam  elevators  are  employed,  and  the 
material  is  carried  on  cars  run  on  to  the  platform  of  the  elevator, 
large  dimensions  must  be  given  to  the  shaft.  Generally  the 
parts  of  the  shaft  used  for  different  purposes  are  separated  by 
partitions.  The  elevator  for  workmen  and  the  various  pipes 
are  placed  in  one  compartment,  while  the  elevator  for  hoisting 
the  excavated  material  and  lowering  construction  material  is 
placed  in  another. 

Shafts  may  be  either  temporary  or  permanent.  They  are 
temporary  when  they  are  filled  in  after  the  tunnel  is  completed, 
and  permanent  when  they  are  left  open  to  supply  ventilation 
to  the  tunnel.  Permanent  shafts  are  usually  made  circular,  and 
lined  with  brick,  unless  excavated  in  very  hard  and  durable 
rock.  When  sunk  for  temporary  use  only,  shafts  are  usually 
made  rectangular  with  the  greater  dimension  transverse  to  the 
tunnel.  They  are  strutted  with  timber.  A  pump  is  generally 
located  at  the  bottom  of  the  shaft  to  collect  the  water  which 
seeps  in  from  the  sides  of  the  shaft  and  from  the  tunnel 
excavation.  The  dimensions  of  this  pump  will  of  course  vary 
with  the  amount  of  water  encountered,  as  will  also  the  capacity 
of  the  pump  for  forcing  it  up  and  out  of  the  shaft,  which  has 
always  to  be  kept  dry. 

The  majority  of  engineers  prefer  to  sink  shafts  directly 
over  the  center  line  of  the  tunnel.  Side  shafts  are  employed 
chiefly  by  French  engineers.  The  chief  advantage  of  the 
former  method  is  the  great  facility  which  it  affords  for  hoisting 
out  the  materials,  while  in  favor  of  the  latter  method  is  the 


38  TUNNELING 

non-interference  of  the  shaft  with  the  operations  inside  the 
tunnel.  Were  it  not  that  the  side  shaft  requires  the  intro- 
duction of  a  transverse  gallery  connecting  it  with  the  tunnel, 
it  would  be  on  the  whole  superior  to  the  center  shaft ;  but  the 
side  gallery  necessitates  turning  the  cars  at  right  angles,  and 
consequently  the  use  of  a  very  sharp  curve  or  a  turntable  to 
reach  the  shaft  bottom,  which  is  a  disadvantage  that  may 
outweigh  its  advantages  in  some  other  respects.  It  is  impos- 
sible to  state  absolutely  which  of  these  methods  of  locating 
shafts  is  the  best ;  both  present  advantages  and  disadvantages, 
and  the  use  of  one  or  the  other  is  usually  determined  more  by 
the  local  conditions  than  by  any  general  superiority  of  either. 

When  side  shafts  are  employed  they  are  sometimes  made 
inclined  instead  of  vertical.  This  form  is  used  when  the  depth 
of  the  shaft  is  small.  By  it  the  hauling  is  greatly  simplified, 
since  the  cars  loaded  at  the  front  with  excavated  material  can 
be  hauled  directly  out  of  the  shaft  and  to  the  dumping-place, 
surmounting  the  inclined  shaft  by  means  of  continuous  cables. 
The  short  galleries  connecting  the  side  shafts  with  the  tunnel 
proper  usually  have  a  smaller  section  than  the  tunnel,  but  are 
excavated  in  exactly  the  same  manner.  Another  form  of  side 
shaft  sometimes  used  is  one  reaching  to  the  surface  when 
the  tunnel  runs  close  to  the  side  of  cliff,  as  is  the  case  with 
some  of  the  Alpine  railway  tunnels. 

CLASSIFICATION  OF  TUNNELS. 

Tunnels  are  classified  in  various  ways,  but  the  most  logical 
method  would  appear  to  be  a  grouping  according  to  the  quality 
of  the  material  through  which  they  are  driven  ;  and  this  method 
will  be  adopted  here.  By  this  method  we  have  first  the  fol- 
lowing general  classification :  (1)  Tunnels  in  hard  rock ;  (2) 
tunnels  in  ordinary  loose  soil;  (3)  tunnels  in  quicksand; 
( 4)  open-cut  tunnels ;  and  (5)  submarine  tunnels.  It  is  hardly 
necessary  to  say  that  this  classification,  like  all  others,  is  simply 


GENERAL,  METHODS  OF  EXCAVATION  39 

an  arbitrary  arrangement  adopted  for  the  sake  of  order  and 
convenience  in  treating  the  subject. 

Tunnels  in  Hard  Rock.  —  With  the  numerous  labor-saving 
methods  and  machines  now  available,  hard  rock  is  perhaps  the 
safest  and  easiest  of  all  materials  through  which  to  drive  a 
tunnel.  Tunnels  through  hard  rock  may  be  excavated,  either 
by  a  drift  or  by  a  heading.  The  difference  depends  upon 
whether  the  advance  gallery  is  located  close  to  the  floor  or 
near  the  soffit  of  the  section. 

Tunnels  in  Loose  Soils.  —  In  driving  tunnels  through  loose 
soils  many  different  methods  have  been  devised,  which  may  be 
grouped  as  follows:  (1)  Tunnels  excavated  at  the  soffit  — 
Belgian  method;  (2)  tunnels  excavated  along  the  perimeter 
—  German  method;  (3)  tunnels  excavated  in  the  whole  sec- 
tion —  English  and  Austrian  methods ;  (4)  tunnels  excavated 
in  two  halves  independent  of  each  other —  Italian  method. 

(1)  Excavating   the  tunnel    by  beginning  at   the  soffit  of 
the  section,  or  by  the  Belgian  method,  is  the  method  of  tunnel- 
ing in  loose  soils  most  commonly  employed  in  Europe  at  the 
present   time.      It   consists    in    excavating    the   soffit   of    the 
section  first ;  then  building  the  arch,  which  is  supported  upon 
the  unexcavated  ground ;  and  finally  in  excavating  the  lower 
portion    of    the    section,    and   building    the    side    walls    and 
invert. 

(2)  In  excavating  tunnels  along  the  perimeter  an  annular 
excavation  is  made,  following  closely  the  outline  of  the  sec- 
tional profile  in  which  the  lining  masonry  is  built,  after  which 
the   center  core   is   excavated.     In    the    German  method   two 
drifts  are  opened  at  each  side  of  the  tunnel  near  the  bottom. 
Other  drifts  are  excavated,  one  above  the  other,  on  each  side 
to  extend  or  heighten  the  first  two  until  all  the  perimeter  is 
open  except  across  the  bottom.     The  masonry  lining  is  then 
built  from  the  bottom  upwards  on  each  side  to  the  crown  of 
the  arch,  and  then  the  center  core  is  removed  and  the  invert 
is  built 


40  TUNNELING 

(3)  This  method,  as  its  name  implies,  consists  in  taking 
out  short  lengths  of  the  whole  sectional  profile  before  begin- 
ning the   building  of   the  masonry.     In  the    English  method 
the  lengths  of   section  excavated  vary  from  10  ft.  to  25  ft. 
The  masonry  invert   is   built  first,   then  the  side  walls,  and 
finally  the  arch.     The  excavators  and  the  masons  work  alter- 
nately,  the   excavation   being    stopped   while   the    masonry  is 
being  built,  and  vice  versa.     The  Austrian  method  differs  in 
two  particulars  from  the  English :  the  length  of  section  opened 
is  made  great  enough  to  allow  the  excavators  to  continue  work 
ahead  of  the  masons,  and   the  side  walls  and  roof  are  built 
before  the  invert. 

(4)  The  Italian  method  is  very  seldom  employed  on  account 
of  its  expensiveness,  but  it  can  often  be  used  where  the  other 
methods  fail.     It  consists  in  excavating  the  lower  half  of  the 
section,  and  building  the  invert  and  side  walls,  and  then  filling 
the    space   between   the    walls  in  again  except  for  a  narrow 
passageway  for  the  cars ;  next  the  upper  part  of  the  section  is 
excavated,  as  in  the  Belgian  method,  and  the  arch  is  built ;  and 
finally  the  soil  in  the  lower  part  is  permanently  removed. 

Tunnels  in  Quicksand.  — -  Tunnels  through  quicksand  are 
driven  by  one  of  the  ordinary  soft-ground  methods  after  drain- 
ing away  the  water,  or  else  as  submarine  tunnels. 

Open-Cut  Tunnels.  —  Open-cut  tunnels  are  those  driven  at 
such  a  small  depth  under  the  surface  that  it  is  more  convenient 
to  excavate  an  open  cut,  build  the  tunnel  masonry  inside  it, 
and  then  refill  the  open  spaces,  than  it  is  to  carry  on  the  work 
entirely  underground.  In  firm  soils  the  usual  mode  of  opera- 
tion is  to  excavate  first  two  parallel  trenches  for  the  side  walls, 
then  remove  the  core,  and  build  the  arch  and  the  invert.  In 
unstable  soils,  since  the  invert  must  be  built  first,  it  is  usual 
to  open  up  a  single  wide  trench.  In  infrequent  cases  where 
a  tunnel  is  desired  in  a  place  which  is  to  be  filled  in,  the 
masonry  is  built  as  a  surface  structure,  which  in  due  time  is 
covered. 


GENERAL  METHODS  OF  EXCAVATION 


41 


Submarine  Tunnels. -- The  mode  of  procedure  followed  in 
excavating  submarine  tunnels  depends  upon  whether  the  mate- 
rial penetrated  is  pervious  or  impervious  to  water.  In  imper- 
vious material  any  of  the  ordinary  methods  of  tunneling  found 
suitable  may  be  employed.  In  pervious  material  the  excava- 
tion may  be  accomplished  either  by  means  of  compressed  air 
to  keep  the  water  out  of  the  excavation,  or  by  means  of  a 
shield  closing  the  front  of  the  excavation,  or  by  a  combination 
of  these  two  methods.  Tunnels  on  the  river  bed  are  built  by 
means  of  coffer  dams  which  inclose  alternate  portions  of  the 
work,  or  by  sinking  a  continuous  series  of  pneumatic  caissons 
and  opening  communication  between  them. 


In  hard  rock,  j 

In  loose  soil.    •< 

METHODS  OF 
EXCAVATING   - 
TUNNELS. 

In  quicksand. 

Open-cut 
tunnels. 

Submarine 
tunnels. 

By  drifts. 

By  a  heading. 

By  upper  half: 
the  arch  is  built  be- 
fore the  side  walls. 

By  the  perimeter: 
excavated    and  lined 
before  the  central 
nucleus  is  battered 
down. 

By  whole  section: 
the  lining  begins  after 
the  whole  section  is 
excavated. 

By  halves: 

the  lower  half  is  ex- 
cavated, lined,  and 
filled  in  again,  fol- 
lowed by  the  work  of 
the  upper  half. 


In  resistant  soils. 

In  loose  soils. 
Built  up. 

At  <rreat  depths  under 
the  river  bed. 

At  small  depths 
under  the  river  bed. 


Belgian  method. 

German  method. 

English  method. 
Austrian  method. 

Italian  method. 


By  two  lateral  nar- 
row trenches. 

By  one  very  large 
trench. 

Bv  slices. 


On  the  river  bed. 


j  By  any  method. 

f  By  shield. 

By  compressed  air. 

By  shield  and  com- 
pressed air. 

By  coffer  dams. 

By  pneumatic  cais- 
sons. 


42  TUNNELING 

The  above  diagram  gives  in  compact  form  the  classifica- 
tion of  tunnels  according  to  materials  penetrated  and  methods 
of  excavation  adopted,  which  have  been  described  more  fully 
in  the  preceding  paragraphs.  It  may  be  noted  here  again  that 
this  is  a  purely  arbitrary  classification,  and  serves  mostly  as  a 
convenience  in  discussing  the  different  classes  of  tunnels  with- 
out confusion. 


TIMBERING    OR    STRUTTING    TUNNELS  43 


CHAPTER   V. 

METHODS    OF   TIMBERING   OR   STRUTTING 
TUNNELS. 


THE  purpose  of  timbering  or  strutting  in  tunnel  work  is  to 
prevent  the  caving-in  of  the  roof  and  side  walls  of  the  exca- 
vation previous  to  the  construction  of  the  lining.  As  the 
strutting  has  to  resist  all  the  pressures  developed  in  the  roof 
and  side  walls,  which  may  be  exceedingly  troublesome  and 
of  great  intensity  in  loose  soils,  its  design  and  erection  call 
for  particular  care.  The  method  of  strutting  adopted  depends 
upon  the  method  of  excavation  employed;  but  in  every  case 
the  problem  is  not  only  to  build  it  strong  enough  to  withstand 
the  pressures  developed,  but  to  do  this  as  economically  as 
possible,  and  with  as  little  hindrance  as  may  be  to  the  work 
which  is  going  on  simultaneously  and  which  will  come  later. 
Only  the  latter  general  problems  of  strutting  peculiar  to  all 
methods  of  tunnel  work  will  be  considered  here.  For  this 
consideration  strutting  may  be  classified  according  to  the 
material  of  which  it  is  built,  under  the  heads  of  timber  struc- 
tures and  iron  structures. 

TIMBER   STRUTTING. 

Timber  is  nearly  always  employed  for  strutting  in  tunnel 
work.  So  long  as  it  has  the  requisite  strength,  any  kind  of 
timber  is  suitable  for  strutting,  since,  it  being  only  temporarily 
employed,  its  durability  is  a  matter  of  slight  importance. 
Timber  with  good  elastic  properties,  like  pine  or  spruce,  is 
preferably  chosen,  since  it  yields  gradually  under  stress,  thus 


44  TUNNELING 

warning  the  engineer  of  the  approach  of  danger  ;  while  oak  and 
other  strong  timbers  resist  until  the  last  moment,  and  then 
yield  suddenly  under  the  breaking  load.  Soft  woods,  moreover, 
are  usually  lighter  in  weight  than  hard  woods,  which  is  a  con- 
siderable advantage  where  so  much  handling  is  required  in 
a  restricted  space.  Round  timbers  are  generally  employed, 
since  they  are  less  expensive,  and  quite  as  satisfactory  in  other 
respects  as  sawed  timbers.  In  the  English  and  Austrian 
methods  of  strutting,  which  are  described  further  on,  a  few 
of  the  principal  struts  are  of  sawed  timbers. 

The  various  timbers  of  the  strutting  are  seldom 
attached    by   framed   joints,  but  wedges    are    used 
to  give   them  the   necessary 
bearing    against    each    other. 
Where  framed  joints  are  eni- 
mng  Tunnel  struts  pioyeti  they  are  made  of  the 

by  Halving.  J 

simplest     form     usuallv     bv 

-U    1     •  ^1          •    •     •  *.-      \  T  TV        -.n     FlG.lO.-Round 

halving  the  joining  timbers,  as  shown  by  Fig.  18.     Timber  Post 


Fig.  19  shows  a  form  of  joint  used  where  round 
posts  carry  beams  of  similar  shape.  The  reason  why 
it  is  possible  to  do  away  with  jointed  connections  to  such  a 
great  extent,  is  that  the  strains  which  the  timbers  have  to 
resist  are  either  compressive  or  bending  strains,  and  because 
the  timbers  are  so  short  that  they  do  not  require  to  be  spliced. 
Strutting  of  Headings.  —  The  method  of  strutting  the  head- 
ing that  is  employed  depends  upon  the  material  through  which 
the  heading  is  driven.  In  solid  rock  strutting  may  not  be 
required  at  all,  or  only  for  the  purpose  of  preventing  the 
fall  of  loose  blocks  from  the  roof,  when  vertical  props  are 
erected  where  required,  or  horizontal  beams  are  inserted  into 
the  s.ide  walls,  as  shown  by  Fig.  20.  These  horizontal  beams 
may  be  used  singly  at  dangerous  places,  or  they  may  be  placed 
from  2  ft.  to  3  ft.  apart  all  along  the  heading.  In  the  latter 
case  they  usually  carry  a  lagging  of  planks,  which  may  be 
placed  at  intervals  or  close  together,  and  filled  above  with 


TLMUEllIXG    OR    STRUTTING    TUNNELS 


45 


stone  in  case  the  roof  of  the  excavation  is  very  unstable. 
Planks  iivsed  in  this  manner  are  usually  called  poling-boards. 
Where  the  side  Avails  as  well  as  the  roof  require  support, 


Fi<;.    20.  — Ceiling    Strutting   for 
Tunnel  Roofs. 


Fir,.  21.  —  Ceiling  Strutting  with   Side 
Post  Supports. 


vertical  side  posts  are  employed  to  carry  the  roof  beams,  as 
shown  by  Fig.  21 ;  and,  when  necessary,  poling-boards  are 
inserted  between  these  posts  and  the  walls  of  the  excavation. 

Frame  Strutting.  —  In  very  loose  soils  not  only  the  roof  and 
side  walls,  but  also  the  floor  of  the  heading  require  strutting. 


FIG.  22.  — Sill,  Side  Post  and  Cap 
Cross  Frame  Strutting. 


FIG.     23.  —  Reinforced     Cross     Frame 
Strutting  for  Treacherous  Materials. 


Iii  these  cases  frame  strutting  is  employed,  as  shown  by  Fig-. 
-'2.  It  consists  simply  of  a  rectangular  frame;  at  the  top 
there  is  a  crown  bar  supported  by  two  vertical  side  posts 


46 


TUNNELING 


setting  on  a  sill  laid  across  the  bottom  of  the  heading.  These 
frames  are  spaced  at  close  intervals,  and  carry  longitudinal 
planks  or  poling-boards.  The  sill  of  the  frame  is  sometimes 
omitted  when  the  soil  is  stable  enough  to  permit  it,  and  in  its 
place  wooden  footing  blocks  are  substituted  to  carry  the  side 
posts.  In  soils  where  the  pressures  are  great  enough  to  bend 
the  crown  bar,  a  secondary  frame  is  employed,  as  shown  by 
Fig.  23,  the  two  inclined  roof  members,  or  rafters,  of  which 
support  the  crown  bar  at  the  center. 

It  is  the  more  common  practice  in  driving  headings  through 
soft  soils  to  use  inclined   poling-boards   to    support  the  roof. 


FIG.  24.  —  Longitudinal  Poling-Board  Sys- 
tem of  Roof  Strutting. 


FIG.  25.  — Transverse  Poling-Board  System 
of  Roof  Strutting. 


Fig.  24  shows  one  method  'of  doing  this.  The  method  of 
operation  is  as  follows :  Assuming  the  poling-boards  a  ahd  b 
to  be  in  place,  and  supported  by  the  frames  A,  B  C\  as  shown, 
the  first  step  in  continuation  of  the  work  is  to  insert  the 
poling-board  c  over  the  crown  bar  of  frame  (7,  and  under  the 
block  m.  .Excavation  is  then  begun  at  the  top,  and  as  fast  as 
the  soil  is  removed  ahead  of  it  the  poling-board  c  is  driven 
ahead  until  its  rear  end  only  slightly  overhangs  the  crown  bar 
of  frame  C.  The  remainder  of  the  face  of  the  heading  is  then 
excavated  nearly  to  the  front  end  of  the  poling-board  c,  and 
another  frame  is  setup.  By  a  succession  of t these  operations 


*t>- 

TIMBERING    OR    STRUTTING    TUNNELS  47 

the  heading  is  advanced.  The  poling-boards  at  the  sides  of 
the  heading  are  placed  in  a  similar  manner  to  the  roof  poling- 
boards.  A  second  method  of  using  inclined  poling-boards  is 
shown  by  Fig.  25.  Here  the  poling-boards  run  transversely, 
and  are  supported  by  the  arrangement  of  timbering  shown. 
The  chief  advantage  of  using  these  inclined  poling-boards, 
particularly  in  the  manner  shown  by  Fig.  24,  is  that  the 
excavators  work  .under  cover  at  all  times,  and  are  thus  safe 
from  falling  fragments  or  sudden  cavings. 

Box  Strutting.  —  In  very  treacherous  soils,  such  as  quick- 
sand, peat,  and  laminated  clay,  box  strutting  is  commonly  em- 
ployed. The  method  of  building  this  strutting  is  to  set  up  at 
the  face  of  the  work  a  rectangular  frame^  and  use  it  as  a  guide 
in  driving  a  lagging  or  boxing  of  horizontal  planks  into  the 
soft  soil  ahead.  These  planks  have  sharp  edges,  and  are  driven 
to  a  distance  of  2  ft.  or  3  ft.  into  the  face  of  the  heading,  so  as 
to  inclose  a  rectangular  body  of  earth.  This  earth  is  excavated 
nearly  to  the  ends  of  the  planks,  and  then  another  frame  is 
inserted  close  up  against'the  new  face  of  the  excavation,  which 
supports  the  planks  so  that  the  remainder  of  the  earth  included 
by  them  may  be  removed.  These  two  frames,  with  their  plank 
lagging,  constitute  a  "  box ; "  and  a  series  of  these  boxes,  one 
succeeding  another,  form  the  strutting  of  the  heading. 

Strutting  the  Face.  —  In  some  cases  it  is  found  necessary 
to  strut  the  face  of  the  heading  in  order  to  prevent  it  from 
caving  in.  This  is  generally  done  by  setting  plank  vertically, 
and  bracing  them  up  by  means  of  inclined  props  whose  feet 
abut  against  the  sill  of  the  nearest  cross  frame.  This  strutting 
is  erected  while  the  workmen  are  placing  the  side  and  roof 
strutting,  and  is  removed  to  permit  excavation. 

Full  Section  Timber  Strutting.  —  For  strutting  the  full  section 
two  forms  of  timbering  are  employed,  known  as  the  polygonal 
system  and  the  longitudinal  system. 

Longitudinal  strutting  consists  of  a  timber  structure  so 
arranged  as  to  have  all  the  principal  members  supporting  the 


48 


TUNNELING 


poling-boards  parallel  to  the  axis  of  the  tunnel.  This  system 
of  strutting  is  peculiar  to  the  English  method  of  tunneling. 
The  longitudinal  timbers  rest  on  this  finished  masonry  at  one 
end,  and  are  carried  on  a  cross  frame  or  by  props  at  the  other 
end.  At  intermediate  points  the  longitudinals  are  braced 
apart  by  struts  in  planes  transverse  to  the  tunnel  axis.  This 
construction  makes  a  very  strong  strutting  framework,  since 
the  transverse  struts  act  as  arch  ribs  to  stiffen  the  longitu- 
dinals; but  the  use  of  transverse  poling-boards  requires  the 
excavation  of  a  larger  cross-section  than  is  necessary  when  longi- 
tudinal poling-boards  are  employed,  and  this  increases  the 
cost  both  for  the  amount  of  earth  excavated  and  the  greater 
quantity  of  filling  required. 

In  polygonal  strutting  the  main  members  are  in  a  plane 
normal  to  the  axis  of  the  tunnel.  They  form  a  polygon  whose 
sides  follow  closely  the  sectional  profile  of  the  excavation. 
These  polygonal  frames  are  placed  at  more  or  less  short  inter- 
vals apart,  and  are  braced  together  by  short  longitudinal  struts 
lying  close  to  the  sides  of  the  excavation,  and  running  from 
one  frame  to  the  next,  and  also  by  longer  longitudinal  members 
which  extend  over  several  frames.  The  polygonal  system  of 
strutting  is  peculiar  to  the  Austrian  method  of  tunneling,  and 
is  fully  described  in  a  succeeding  chapter.  One  of  its  distinc- 
tive characteristics  is  that 
the  poling-boards  are  in- 
serted parallel  to  the  tunnel 
axis.  Polygonal  strutting 
is  generally  held  to  be 
stronger  than  longitudinal 
strutting  under  uniform 
loads,  but  it  is  more  liable 
to  distortion  when  the 
loads  are  unsym metrical. 
Strutting  of  Shafts.  —  Tunnel  shafts  are  strutted  both  to 
prevent  the  caving-in  of  the  sides  and  to  divide  them  into 


Fin.   26.  — Shaft  with    Single  Transverse 
Strutting. 


TIMBERING    OK    STRUTTING    TUNNELS 


49 


compartments.  When  the  material  penetrated  is  very  compact, 
and  caving  is  not  likely,  a  single  series  of  transverse  struts,  one 
above  the  other,  running  from  the  top  to  the  bottom  of  the 
shaft,  as  shown  by  Fig.  26,  is  used  to  divide  it  into  two  com- 
partments. In  softer  material,  where  the  sides  of  the  shaft 
require  support,  Fig.  27 
shows  a  form  of  strutting 
commonly  employed.  It 
consists  of  vertical  corner 
posts  braced  apart  at  inter- 
vals by  four  horizontal  struts 
placed  close  to  the  walls  of 
the  shaft.  The  longer  side 

0  FIG.  27.— Rectangular  Frame  Strutting  for  Shafts. 

struts  are  also  braced  apart 

at  the  center  by  a  middle  strut  which  divides  the  shaft  into 
two  compartments.  A  lagging  of  vertical  plank  is  placed 
between  the  walls  of  the  shaft  and  the  horizontal  side  struts. 
In  very  loose  soils  the  form  of  strutting  shown  by  Fig.  28  is 
employed.  This  is  practically  the  same  construction  as  is 
shown  by  Fig.  27,  with  the  addition  of  an  interior  polygonal 

horizontal  bracing  in  each 
half  of  the  shaft.  Referring 
to  Fig.  28,  the  timbers  #,  a, 
etc.,  are  vertical  and  con- 
tinuous from  the  top  to  the 
bottom  of  the  shaft;  and 
the  horizontal  timbers,  ft,  ft, 
etc.,  are  spaced  at  more  or 
less  close  intervals  verti- 
cally. The  lagging  plank 

may  be  laid  with  spaces  between  them,  or  close  together,  or, 
in  case  of  very  loose  material,  with  their  edges  overlapping. 
The  manner  of  constructing  the  strutting  is  also  governed  by 
the  stability  of  the  soil.  In  firm  soils  it  is  possible  to  sink  the 
shaft  quite  a  depth  without  timbering,  and  the  timbering  can 


FIG.  28.  —  Reinforced  Rectangular  Frame  Strut- 
ting for  Shafts  in  Treacherous  Materials. 


50  TUNNELING 

be  erected  in  sections  of  considerable  length,  which  is  always 
an  advantage,  but  in  loose  soils  the  timbering  has  to  follow 
closely  the  excavation. 

The  solid  wall  shaft  struttings  which  have  been  described 
are  discontinued  at  the  point  where  the  shaft  intersects  the 
tunnel  excavation ;  and  from  this  point  to  the  floor  of  the 
tunnel  an  open  timbering  is  employed,  whose  only  duty  is  to 
support  the  weight  of  the  solid  strutting  above.  This  timber- 
ing is  made  in  various  forms,  but  the  most  common  is  a  timber 
truss  or  arch  construction  which  spans  the  tunnel  section. 

Quantity  of  Timber.  —  The  quantity  of  timber  employed  in 
strutting  a  tunnel  varies  with  the  character  of  the  material 
through  which  the  tunnel  is  excavated :  it  is  small  for  solid- 
rock  tunnels,  and  large  for  soft-ground  tunnels.  In  the  Bel- 
gian method  of  excavation  a  smaller  quantity  of  timber  is 
used  than  in  any  of  the  other  ordinary  methods.  For  single- 
track  tunnels  excavated  by  this  method  there  will  be  needed 
on  an  average  about  3  to  3J  cu.  yds.  of  timber  per  lineal  foot 
of  tunnel.  Practical  experience  shows  that  about  four-fifths  of 
the  timber  once  used  can  be  employed  for  the  second  time. 
In  any  of  the  methods  in  which  the  whole  tunnel  section  is 
excavated  at  once,  the  average  amount  of  timber  required  per 
lineal  foot  is  about  8.7  cu.  yds.  Of  this  amount  about  two- 
thirds  can  be  used  a  second  time.  In  the  Italian  method,  in 
which  the  upper  half  and  the  lower  half  are  excavated  separately, 
about  5  cu.  yds.  of  timber  are  required  per  lineal  foot  of  tunnel, 
about  one-half  of  which  can  be  employed  a  second  time.  For 
quicksand  tunnels  the  amount  of  timbering  required  per  lineal 
foot  varies  from  3  to  5  cubic  yds.  Shaft  strutting  requires 
from  1  to  1-j-  cu.  yds.  of  timber  per  lineal  foot. 

Dimensions  of  Timber.  —  The  dimensions  of  the  principal  mem- 
bers composing  the  strutting  of  headings,  full  section,  and 
shafts,  are  given  in  Table  I.  The  planks  used  for  lagging 
or  the  poling-boards  are  usually  from  4  ins.  to  6  ins.  wide, 
with  a  length  depending  upon  the  method  of  strutting  employed. 


TIMBERING    OB    STRUTTING   TUNNELS 
TABLE    I. 


51 


Showing  Sizes  of  Various  Timbers  Used  in  Strutting  Tunnels  Driven 
Through  Different  Materials. 


Headings  : 
Cap-pieces  and  vertical  struts 

ROCK. 

SOFT  SOILS. 

1 

ins. 
6 

5 
6 

12 

10 
8 
10 
6 
4.5 

8 
10 
12 
12 

10 
6 

8 
8 
8 

6 

!_ 

ins. 
8 

5 
4.5 

14 

12 
8 
12 
8 
4 

10 
12 
14 
14 

10 
4.5 

8 
8 
8 
6 
4.5 

5  "S 

st 

ins. 

10 

8 
6 
3 

14 

14 
10 
14 
10 
3 

12 
14 
16 
16 
12 
10 
4 

10 
8 
10 
8 
4 

I 

>•>    c 

*  g 

>  JS 
Inir 

14 
12 
8 
2.6 

16 
18 
24 
24 
18 
12 
3 

14 
12 
12 
8 
2.6 

ins. 

12 
10 
7 
2.6 

14 
16 
20 
20 
16 
12 
3 

12 
10 

12 
8 
3 

Sills        

Distance  apart  of  the  frames  in  feet          .     .     . 
Strutting  of  the  tunnel,  longitudinal  strutting  : 

Props  vertical  or  inclined  supporting  the  crown 

Sills    

Cap-pieces  or  saddles    

Struts  to  stiffen  the  structure      

Distance  apart  of  the  frames  (in  feet)       .     .     . 
Polygonal  strutting  : 

Vertical  struts  on  top    

Vertical  struts  below     .... 

Intermediate  sills      

Rakinf  props  

Distance  apart  of  the  frames  (in  feet)       .     .     . 
Shafts  : 
Horizontal  beams  forming  the  frame    .... 
Transverse  beams     

Vertical  struts  between  the  frames  
Struts  to  reenforce  the  frame      

Distance  apart  of  the  strutting  (in  feet)    .     .     . 

IRON   STRUTTING. 

In  1862  Mr.  Rziha  employed  old  iron  railway  rails  for 
strutting  the  Naenesn  tunnel,  and  his  example  was  successfully 
followed  in  several  tunnels  built  later  where  timber  was  scarce 


52  TUNNELING 

and  expensive.  The  advantages  which  iron  strutting  is  claimed 
to  possess  over  the  more  common  wooden  structure  are :  its 
greater  strength ;  the  smaller  amount  of  space  which  it  takes 
up ;  and  the  fact  that  it  does  not  wear  out,  and  may,  therefore, 
be  used  over  and  over  again. 

Iron  Strutting  in  Headings.  —  In  strutting  the  headings  the 
cross  frames  have  a  crown  bar  consisting  of  a  section  of  old 
railway  rail  carried  either  by  wood  or  iron  side  posts.  When 

wooden  side  posts  are  used  their 

upper  ends  have  a  dovetail  mor- 
tise, and  are  bound  with  an  iron 

band,  as  shown  by  Fig.  29.     The 

base  of  the  rail  crown  bar  is  set 

into    the    dovetail     mortise    and 

fastened  by  wedges.     When  iron 

FIG.  29. -Strut-      Slde     P°StS      are      emPloyed     theJ     FIG.    30.  -  Strutting- 

ting  of  Timber     usually  consist  of  sections  of  rail-      made  entirely  of 

PostsandRail-  .,  ,          .  Railway  Rails, 

way  Rail  Caps.      waJ    rails»   and    the    crown    bar   1S 

attached  to  them  by  fish-plate  connections,  as 
shown  by  Fig.  30.  The  iron  cross  frames  are  set  up  as  the 
heading  advances,  and  carry  the  plank  lagging  or  poling-boards, 
exactly  in  the  same  manner  as  the  timber  cross  frames  previ- 
ously described. 

Full  Section  Iron  Strutting-.  —  The  iron  strutting  devised  by 
Mr.  Rziha  for  full  section  work  is  shown  by  Fig.  31.  Briefly 
described,  it  consists  of  voussoir-shaped  cast-iron  segments, 
which  are  built  up  in  arch  form.  Fig.  32  shows  the  construc- 
tion of  one  of  the  segments,  all  of  which  are  alike,  with  the 
exception  of  the  crown  segment,  which  has  a  mortise  and 
tenon  joint  which  is  kept  open  by  filling  the  mortise  with  sand. 
The  segments  are  bolted  together  by  means  of  suitable  bolt- 
holes  in  the  vertical  flanges,  and  when  fully  connected  form  an 
arch  rib  of  cast  iron.  This  arch  rib,  A,  Fig.  31,  carries  a  series 
of  angle  or  T-iron  frames  bent  into  approximately  voussoir 
shape,  as  shown  at  B,  Fig.  31.  Above  these  frames  are  inserted 


TIMBERING    OR    STRUTTING    TUNNELS 


53 


FIG.  31.  —  Rziha's  Combined  Strutting  and  Centering 
of  Cast  Iron. 


the    poling-boards,  running    longitudinally,  and   spanning    the 
distance  between  consecutive  arch  ribs.     By  removing  the  bent 
iron  frames  the  cast-iron  rib  forms  a  center  upon  which  to  con- 
struct the  masonry.     Fi- 
nally, to  remove  the  cast- 
iron  rib  itself,  the   sand 
is  drawn  out  of  the  mor- 
tise   and  tenon   joint  in 
the  crown  segment,  which 
allows  the  joint  to  close, 
and  loosen  the  segments 
so   that   they  are   easily 
unbutted. 

The  illustration,  Fig. 
31,  shows  longitudinal 
poling-boards;  more  often 
longitudinal  crown  bars 
of  railway  rails  span  the  space  between  connective  arch  ribs, 
and  support  transverse  poling-boards.  In  building  the  masonry, 
work  is  begun  at  the  bottom  on  each  side,  the  bent  iron  frames 
being  removed  one  after  another  to  give  room  for  the  masonry. 

As  each  frame  is  removed,  it  is 
replaced  with  a  sort  of  screw  jack- 
screw  to  support  the  poling-boards 
until  the  masonry  is  sufficiently 
completed  to  allow  their  removal. 
The  interior  bracing  of  the  arch 
rib  shown  at  a  a  and  b  b  consists 
of  railway  rails  carried  by  brack- 
ets cast  on  to  the  segments.  A 
similar  bracing  of  rails  connects  the  successive  arch  ribs.  These 
lines  of  bracing  serve  to  carry  the  scaffolding  upon  which  the 
masons  work  in  building  the  lining. 

Iron  Shaft  Strutting.  —  In  soft-ground  shaft  work,  the  use  of 
an  iron  strutting,  consisting  of  consecutive   cast-iron  rings,  has 


Pro.  32.  —  Cast-iron  Segment  of  Rziha's 
Strutting  and  Centering. 


TUNNELING 


sometimes  been  employed  to  advantage.  Fig.  33  shows  the 
construction  of  one  of  these  rings,  which,  it  will  be  seen,  is  com- 
posed of  four  segments  connected  to  each  other  by  means  of 
bolted  flanges.  The  holes  shown  in  the  circumferential  web  of 
the  ring  are  to  allow  for  the  seepage  from  the  earth  side  walls. 

The  method  of  placing  this 
cylindrical  strutting  is  to 
start  with  a  ring  having  a 
cutting-edge.  By  means 
of  excavation  inside  the 
ring,  and  by  ramming, 


the  ring  is  sunk  into  the 
ground  a  distance  equal  to 
its  height.  Another  ring 
is  then  fastened  by  special  hooks  on  top  of  the  first  one,  and 
the  sinking  continued  until  the  second  ring  is  down  flush  with 
the  surface.  A  third  ring  is  then  added,  and  so  on  until  the 
entire  shaft  is  excavated  and  strutted.  As  in  timber  shaft 
strutting,  the  solid  iron  ring  strutting  is  carried  down  only  to 
the  top  of  the  tunnel  section,  and  below  this  point  there  is  an 
open  timber  or  iron  supporting  framework. 


FIG.  33.  — Cast-iron  Segmental  Strutting  for 
Shafts. 


METHODS    OF    HAULING    IN    TUNNELS 


55 


CHAPTER   VI. 
METHODS    OF    HAULING    IN    TUNNELS. 


THE  transportation  from  one  point  to  another  within  the 
tunnel  and  its  shafts  of  any  material,  whether  it  is  excavated 
spoil  or  construction  material,  is  defined  as  hauling.  In  all 
engineering  construction,  the  transportation  of  excavated 
materials,  and  materials  for  construction,  constitutes  a  very 
important  part  of  the  expense  of  the  work;  but  hauling  in 
tunnels  where  the  room  is  very  limited,  and  where  work  is 
constantly  in  progress  over  and  at  the  sides  of  the  track,  is  a 
particularly  expensive  process.  Hauling  in  tunnels  may  be 
done  either  by  way  of  the  entrances,  or  by  way  of  the  shafts, 
or  by  way  of  both  the  entrances  and  shafts. 

Hauling  by  Way  of  Entrances.  —  When  the  hauling  is  done 
by  the  way  of  the  entrances,  the  materials  to  be  hauled  are 
taken  directly  from  the  point 
of  construction  to  the  en- 
trances, or  in  the  opposite  di- 
rection, by  means  of  special 
cars  of  different  patterns.  For 
general  purposes,  these  differ- 
ent patterns  of  cars  may  be 
grouped  into  three  classes,  — 
platform-cars,  dump-cars,  and 
box-cars.  Representative  ex-  FlG.  ^  _  P^tform  Car  for  Tunnel  Work. 
amples  of  these  several  classes 

of  cars  are  shown  in  Figs.  34  to  37  *  inclusive,  but  it  will  be 
readily  understood  that  there  are  many  other  forms. 

Briefly    described,    platform-cars    (Fig.    34)    consist   of    a 

*  Reproduced  from  catalogue  of  Arthur  Koppel,  New  York. 


56 


TUNNELING 


FIG.  35.  —Iron  Dump-Car  for 
Tunnel  Work. 


wooden  platform  mounted  on  tracks,  and  they  are  usually  em- 
ployed for  the  transportation  of  timber,  ties,  etc.  Dump-cars 
are  used  in  greater  numbers  in  tunnel  work  than  any  other 
form.  Fig.  35  shows  a  dump-car  of  metal  construction,  and 

Fig.  36  one  constructed  with  a  metal 
under-frame  t*nd  wooden  box.  These 
cars  are  made  to  run  on  narrow-gauge 
tracks,  and  usually  have  a  capacity  of 
about  one  to  one  and  one-half  cubic 
yards.  Box-cars  are  more  extensively 
employed  in  Europe  for  tunnel  work 
than  in  America.  Fig.  37  shows  a 
typical  European  box-car  for  tunnel 
work.  It  is  made  either  to  run  on  narrow-gauge  or  standard- 
gauge  tracks. 

It  is  usually  desirable  in  tunnel  work  to  employ  cars  of 
different  forms  for  different  parts  of  the  work.  In  rock 
tunnels  it  is  a  common  practice  to  use  narrow-gauge  cars  of 
small  size  in  the  headings,  arid 
larger,  broad-gauge  cars  for  the 
enlargement  of  the  profile. 
Where  narrow-gauge  cars  are 
employed  for  all  purposes,  it  will 
also  be  found  more  convenient 
to  use  platform-cars  for  handling 
the  construction  material,  and 
dump-cars  for  removing  the  spoil. 
The  extent  to  which  it  is  desir- 
able tO  USe  Cars  Of  different  forms  FIG.  36.  — Wooden  Dump-Car  for  Tunnel 

will  depend  upon    the   character 

and  conditions  of  the  work,  and  particularly  upon  how  far  it  is 

possible  to  install  the  permanent  track. 

As  a  general  rule,  it  is  considered  preferable  to  lay  the 
permanent  tracks  at  once,  and  do  all  the  hauling  upon  them, 
so  that  as  soon  as  the  tunnel  is  completed,  trains  may  pass 


METHODS    OF    HAULING    IX    TUNNELS  57 

through  without  delay.  To  what  extent  this  may  be  done,  or 
whether  it  can  be  done  at  all  or  not,  depends  upon  the  method 
of  excavation  and  other  local  conditions.  In  soft-ground 
tunnels  excavated  by  the  English  or  Austrian  methods, 
it  is  quite  possible  to  lay  the  permanent  tracks  at  first,  since 
the  whole  section  is  excavated  at  once,  and  the  excavation  is 
kept  bat  a  little  ahead  of  the  complete/1  tunnel.  In  rock 
tunnels,  where  the  heading  is  driven  far  ahead  of  the  com- 
pleted section,  it  is,  of  course,  impossible  to  keep  the  perma- 
nent track  close  to  the  advance  work,  and  narrow-gauge  tracks 
must  be  laid  in  the  heading.  The  same  thing  is  true  in  soft- 
ground  tunnels  driven  by  successive  headings  and  drifts.  In 
these  cases,  therefore, 
where  narrow-gauge 
tracks  have  to  be  used 
for  some  portions  of 
the  work  anyway,  the 
question  comes  up 
whether  it  is  preferable 

FIG.  37  —  Box-Car  for  Tunnel  Work. 

to    use     temporary 

narrow-gauge  tracks  throughout,  or  to  lay  the  permanent  track 
as  far  ahead  as  possible,  and  then  extend  narrow-gauge  tracks 
to  the  advance  excavation.  In  the  latter  case  it  will,  of  course, 
be  necessary  to  trans-ship  each  load  from  the  narrow-gauge  to 
the  standard-gauge  cars,  or  vice  versa,  which  means  extra  cost 
and  trouble.  To  avoid  this,  the  method  is  sometimes  adopted 
of  laying  a  third  rail  between  the  standard-gauge  rails,  so  that 
either  standard-  or  narrow-gauge  cars  may  be  transported  over 
the  line.  Whatever  form  the  local  conditions  may  require  the 
system  of  construction  tracks  to  assume,  it  may  be  set  down  as 
a  general  rule  that  the  permanent  tracks  should  be  kept  as  far 
advanced  as  possible,  and  temporary  tracks  employed  only 
where  the  permanent  tracks  are  impracticable. 

The  motive  power  employed  for  hauling  in  tunnels  may  be 
furnished  by  animals  or  by  mechanical  motors.     Animal  power 


58  TUNNELING 

is  generally  employed  in  short  tunnels  and  in  the  advance 
headings  and  galleries.  In  long  tunnels,  or  where  the  exca- 
vated material  has  to  be  transported  some  distance  away  from 
the  tunnel,  mechanical  power  is  preferable,  for  obvious  reasons. 
The  motors  most  used  are  small  steam  locomotives,  special 
compressed-air  locomotives,  and  electric  motors.  Compressed 
air  and  electric  locomotives  are  built  in  various  forms,  and  are 
particularly  well  adapted  for  tunnel  work  because  of  their 
small  dimensions,  and  freedom  from  smoke  and  heat. 

Hauling  by  Way  of  Shafts.  —  When  the  excavated  material 
and  materials  of  construction  are  handled  through  shafts,  the 
operation  of  hauling  may  be  divided  into  three  processes : 
the  transportation  of  the  materials  along  the  floor  of  the 
tunnel,  the  hoisting  of  them  through  the  shaft,  and  the  sur- 
face transportation  from  and  to  the  mouth  of  the  shaft.  These 
three  operations  should  be  arranged  to  work  in  harmony  with 
each  other,  so  as  to  avoid  waste  of  time  and  unnecessary  han- 
dling of  the  materials.  An  endeavor  should  be  made  to  avoid, 
if  possible,  breaking  or  trans-shipping  the  load  from  the  time 
it  starts  at  the  heading  until  it  is  dumped  at  the  spoil  bank. 
This  can  be  accomplished  in  two  ways.  One  way  is  to  hoist 
the  boxes  of  the  cars  from  their  trucks  at  the  bottom  of  the 
shaft,  and  place  them  on  similar  trucks  running  on  the  surface 
tracks.  The  other  way  is  to  run  the  loaded  cars  on  to  the  ele- 
vator platform  at  the  bottom,  hoist  them,  and  then  run  them 
on  to  the  surface  tracks.  If  the  first  method  is  employed,  the 
car  box  is  usually  made  of  metal,  and  is  provided  at  its  top 
edges  with  hooks  or  ears  to  which  to  attach  the  hoisting  cables. 
When  the  second  method  is  used,  the  elevator  platform  has 
tracks  laid  on  it  which  connect  with  the  tracks  on  the  tunnel 
floor,  and  also  with  those  on  the  surface. 

Hoisting  Machinery.  —  The  machines  most  commonly  em- 
ployed for  hoisting  purposes  in  tunnel  shafts  are  steam  hoisting 
engines,  horse  gins,  and  windlasses  operated  by  hand.  Wind- 
lasses and  horse  gins  are  rather  crude  machines  for  hoisting- 


METHODS    OF    HAULING    IN    TUNNELS  59 

loads,  and  are  used  only  in  special  circumstances,  where  the 
shaft  is  of  small  depth,  when  the  amount  of  material  to  be 
hoisted  is  small,  or  where  for  any  reason  the  use  of  hoisting 
engines  is  precluded.  The  steam  hoisting  engine  is  the  stan- 
dard machine  for  the  rapid  lifting  of  heavy  vertical  loads. 
Recently  oil  engines  and  electric  hoists  have  also  come  to  be 
used  to  some  extent,  and  under  certain  conditions  these  ma- 
chines possess  notable  advantages. 

The  construction  of  hand  windlasses  is  familiar  to  every  one. 
In  tunnel  work  this  device  is  located  directly  over  the  shaft, 
with  its  axis  a  little  more  than  half  a  man's  height,  so  that  the 
crank  handle  does  not  rise  above  the  shoulder  line.  To  develop 
its  greatest  efficiency  the  hoisting  rope  is  passed  around  the 
windlass  drum  so  that  the  two  ends  hang  down  the  shaft,  and 
as  one  end  descends  the  other  ascends.  A  skip,  or  bucket,  is 
attached  to  each  of  the  rope  ends ;  and  by  loading  the  descend- 
ing skip  with  construction  materials  and  the  ascending  skip 
with  spoil,  the  two  skip  loads  tend  to  balance  each  other,  thus 
increasing  the  capacity  of  the  windlass,  and  decreasing  the 
manual  labor  required  to  operate  it.  Skips  varying  from  0.3 
cu.  yd.  to  0.5  cu.  yd.  are  used.  The  horse  gin  consists  of  a 
vertical  cylinder  or  drum  provided  with  radial  arms  to  which 
the  horses  are  hitched,  which  revolve  the  cylinder  by  walking 
around  it  in  a  circle.  The  hoisting  rope  is  rove  around  the 
drum  so  that  the  two  ends  extend  down  the  shaft  with  skips 
attached,  as  described  in  speaking  of  the  hand  windlass.  The 
power  of  the  horse  gin  is,  of  course,  much  greater  than  that  of  a 
windlass  operated  by  hand,  skips  of  1  cu.  yd.  capacity  being 
commonly  used.  Horse  gins  are  no  longer  economical  hoisting 
machines,  according  to  one  prominent  authority,  when  V 
(H+20)  >  5000,  where  V  equals  the  volume  of  material  to 
be  hoisted,  and  H  equals  the  height  of  the  hoist,  the  weight  of 
the  excavated  material  being  2100  Ibs.  per  cu.  yd.  As  a  gen- 
eral rule,  however,  it  is  assumed  that  it  is  not  economical  to 
employ  horse  gins  with  a  depth  of  shaft  exceeding  150  ft. 


60  TUNNELING 

As  already  stated,  the  most  efficient  and  most  commonly 
used  device  for  hoisting  at  tunnel  shafts  is  the  steam  hoisting 
engine.  There  are  numerous  builders  of  hoisting  engines,  each 
of  which  manufactures  several  patterns  and  sizes  of  engines. 
In  each  case,  however,  the  apparatus  consists  of  a  boiler  supply- 
ing steam  to.  a  horizontal  engine  which  operates  one  or  more 
rope  drums.  The  engines  are  always  reversible.  They  may 
be  employed  to  hoist  the  skips  directly,  or  to  operate  elevators 
upon  which  the  skips  or  cars  are  loaded.  In  either  case  the 
hoisting  ropes  pass  from  the  engine  drum  to  and  around  ver- 
tical sheaves  situated  directly  over  the  shaft  so  as  to  secure  the 
necessary  vertical  travel  of  the  ropes  down  the  shaft.  Where 
the  shaft  is  divided  into  two  compartments,  each  having  an  ele- 
vator or  hoist,  double-drum  engines  are  employed,  one  drum 
being  used  for  the  operations  in  one  compartment,  and  the  other 
for  the  operations  in  the  other  compartment.  Where  the  work 
is  to  be  of  considerable  duration,  or  when  it  is  done  in  cold 
weather,  more  or  less  elaborate  shelters  or  engine  houses  are 
built  to  cover  and  protect  the  machinery. 

Choice  between  the  method  of  hoisting  the  skips  directly, 
and  the  method  of  using  elevators,  depends  upon  the  extent  and 
character  of  the  work.  Where  large  quantities  of  material  are 
to  be  hoisted  rapidly,  it  is  generally  considered  preferable  to 
employ  elevators  instead  of  hoisting  the  skips  directly.  In 
direct  hoisting  at  high  speed,  oscillations  are  likely  to  be  pro- 
duced which  may  dash  the  skips  against  the  sides  of  the  shaft 
and  cause  accidents.  The  loads  which  can  be  carried  in  single 
skips  are  also  smaller  than  those  possible  where  elevators  are 
used ;  and  this,  combined  with  the  slower  hoisting  speed  required, 
reduces  the  capacity  of  this  method,  as  compared  with  the  use 
of  elevators.  Where  elevators  are  employed,  however,  the  plant 
required  is  much  more  extensive  and  costly ;  it  comprising  not 
only  the  elevator  cars  with  their  safety  devices,  etc.,  but  the 
construction  of  a  guiding  framework  for  these  cars  in  the  tun- 
nel shaft.  For  these  various  reasons  the  elevator  becomes  the 


METHODS    OF   HAULING   IN    TUNNELS 


61 


preferable  hoisting  device  where  the  quantity  of %  material  to  be 
handled  is  large,  where  the  shafts  are  deep,  and  where  the  work 
will  extend  over  a  long  period  of  time  ;  but  when  the  contrary 
conditions  are  the  case,  direct  hoisting  of  the  skips  is  generally 
the  cheaper.  The  engineer  has  to  integrate  the  various  factors- 
in  each  individual  case,  and 
determine  which  method  will 
best  fulfill  his  purpose,  which 
is  to  handle  the  material  at 
the  least  cost  within  the 
given  time  and  conditions. 
The  construction  of  ele- 
vators for  tunnel  work  is 
simple.  The  elevator  car 
consists  usually  of  an  open 
framework  box  of  timber  and 
iron,  having  a  plank  floor  on 
which  car  tracks  are  laid, 
and  its  roof  arranged  for 
connecting  the  hoisting  cable 

O  O 

(Fig.  38  *).  Rigid  construc- 
tion is  necessary  to  resist  the 
hoisting  strains.  The  sides 
of  the  car  are  usually  de- 
signed to  slide  against  tim- 
ber guides  on  the  shaft  walls. 
Some  form  of  safety  device, 
of  which  there  are  several  kinds,  should  be  employed  to  pre- 
vent the  fall  of  the  elevator,  in  case  the  hoisting  rope  breaks, 
or  some  mishap  occurs  to  the  hoisting  machinery,  which  en- 
dangers the  fall  of  the  car.  Speaking  tubes  and  electric-bell 
signals  should  also  be  provided  to  secure  communication  be- 
tween the  top  and  bottom  of  the  shaft. 

*  Reproduced  from  the  catalogue  of  the  Ledgerwood  Manufacturing  Company,  New 


FIG.  38.  —  Elevator  Car  for  Tunnel  Shafts. 


York. 


62  TUNNELING 


CHAPTER    VII. 

TYPES    OF    CENTERS    AND     MOLDS     EMPLOYED 

IN    CONSTRUCTING    TUNNEL    LININGS 

OF    MASONRY. 


THE  masonry  lining  of  a  tunnel  may  be  described  as  con- 
sisting of  two  or  more  segments  of  circular  arches  combined 
so  as  to  form  a  continuous  solid  ring  of  masonry.  To  direct 
the  operations  of  the  masons  in  constructing  this  masonry 
ring,  templates  or  patterns  are  provided  which  show  the  exact 
dimensions  and  form  of  the  sectional  profile  which  it  is  de- 
sired to  secure.  These  patterns  or  templates  will  vary  in 
number  and  construction  with  the  form  of  lining  and  the 
method  of  excavation  adopted.  Where  the  excavation  is  fully 
lined  on  all  four  sides,  the  masonry  work  is  usually  divided 
into  three  parts,  —  the  invert  or  floor  masonry,  the  side-wall 
masonry,  and  the  roof-arch  masonry.  At  least  one  separate 
pattern  has  to  be  employed  in  constructing  each  of  these  parts 
of  the  lining;  and  they  are  known  respectively  as  ground 
molds,  leading  frames,  and  arch  centers,  or  simply  centers.  In 
the  following  paragraphs  the  form  and  construction  usually 
employed  for  each  of  these  three  kinds  of  patterns  is  de- 
scribed. 

Ground  Molds.  —  Ground  molds  are  employed  in  building  the 
tunnel  invert.  They  are  generally  constructed  of  3-inch  plank 
cut  exactly  to  the  form  and  dimensions  of  the  invert  masonry 
as  shown  in  Fig.  39.  To  permit  of  convenience  of  handling  in 
a  restricted  space,  they  are  generally  made  in  two  parts,  which 
are  joined  at  the  middle  by  means  of  iron  fish-plates  and  bolts. 
Either  one  or  two  ground  molds  may  be  employed.  Where  two 


TYPES  OF  CENTERS  AND  MOLDS 


63 


molds  are  used  they  are  set  up  a  short  distance  apart,  and  cords 
are  stretched  from  one  to  the  other  parallel  to  the  axis  of  the 
tunnel,  by  which  the  masons  are  guided  in  their  work.  Ex- 
treme care  has  to  be  taken  in 
setting  the  molds  to  ensure  that 
they  are  fixed  at  the  proper 
grade,  and  are  in  a  plane  normal  FIG.  39.— Ground  Moid  for  constructing 

,  -  , ,  ,        -fTTi  Tunnel  Invert  Masonry. 

to  the  axis  of  the  tunnel.     Where 

only  one  ground  mold  is  employed,  the  finished  masonry  is 
depended  upon  to  supply  the  place  of  the  second  mold,  cords 
being  stretched  from  it  to  the  single  mold  placed  the  requisite 
distance  ahead.  The  leveling  and  centering  of  the  molds  is  ac- 
complished by  means  of  transit  and  level. 

Two   modifications  of  the   form  of  ground  mold  shown  by 
Fig.   39  are   employed.     The  first  modification  is  peculiar  to 

the  English  method  of 
excavation,  and  consists 
in  combining  the  ground 
mold  with  the  leading 
frame  for  the  lower  part 
of  the  side  walls,  as 


shown  by  Fig.  40.     The 
second    modification    is 

FIG.  40.  -Combined  Grotmd  Mold  and  Leading  Frame    enir>love(J  wherp  the  two 
for  Invert  and  Side  Wall  Masonry.  mpioye 

halves  or  sides  of   the 

invert  are  built  separately,  and  it  consists  simply  in  using  one- 
half  of  the  mold  shown  by  Fig.  39.  When  the  last  method  of 
constructing  the  invert  masonry  is  resorted  to,  extreme  care  has 
to  be  observed  in  setting  the  half-mold  in  order  to  avoid  error. 

Leading  Frames.  —  As  already  stated,  leading  frames  are  the 
patterns,  or  molds,  used  in  building  the  side  walls  of  the  lining. 
Like  the  ground  mold  they  are  usually  built  of  plank  ;  one 
side  being  cut  to  the  curve  of  the  profile,  and  the  other  being 
made  parallel  to  the  vertical  axis  of  the  tunnel  section.  The 
vertical  side  usually  has  some  arrangement  by  which  a  plumb 


64  TUNNELING 

bob  can  be  attached,  as  shown  by  Fig.  41,  to  guide  the  work- 
men in  erecting  the  frame.  The  combined  leading  frame  and 
ground  mold  shown  in  Fig.  40  has  already  been  described. 
The  use  of  this  frame  is  possible  only  where  the- 
masonry  is  begun  at  the  invert  and  carried  up  on 
each  side  at  the  same  time.  This  mode  of  con- 
struction is  peculiar  to  the  English  method  of 
tunneling  ;  in  all  other  methods  the  form  of  sep- 
arate ground  frame  shown  by  Fig.  41  is  employed. 
FIG.  4i.—  Lead-  ^Q  ground  frames  are  lined  in  and  leveled  up  by 

ing  Frame  for  _  * 

constructing      transit  and  level;  and,  as  in  setting  the  ground 


frames,  care  must  be  taken  to  secure  accuracy  in 
both  direction  and  elevation. 

Arch  Centers.  —  The  template  or  form  upon  which  the  roof 
arch  is  built  is  called  a  center.  Unlike  the  ground  molds  and 
leading  frames,  the  arch  centers  have  to  support  the  weight  of 
the  masonry  and  the  roof  pressures  during  the  construction  of 
the  lining,  and  they,  therefore,  require  to  be  made  strong. 
Owing  to  the  fact  that  the  pressures  are  indeterminate,  it  is 
impossible  to  design  a  rational  center,  and  resort  is  had  to  those 
constructions  which  past  experience  has  shown  to  work  satis- 
factorily under  similar  conditions.  In  a  general  way  it  can 
always  be  assumed  that  the  construction  should  be  as  simple 
as  possible,  that  the  center  should  be  so  designed  that  it  can 
be  set  up  and  removed  with  the  least  possible  labor,  and  that 
the  different  pieces  of  the  framework  and  lagging  should  be  as 
short  as  possible,  for  convenience  in  handling. 

Tunnel  centers  are  usually  composed  of  two  parts,  —  a  mold 
or  curved  surface  upon  which  the  masonry  rests,  and  a  frame- 
work which  supports  the  mold.  The  curved  surface  or  mold 
consists  of  a  lagging  of  narrow  boards  running  parallel  to  the 
tunnel  axis,  which  rests  upon  the  arched  top  members  of  two 
or  more  consecutive  supporting  frames.  The  supporting  frame 
is  built  in  the  form  of  a  truss,  and  must  be  made  strong  enough 
to  withstand  the  heavy  superimposed  loads,  consisting  of  the 


TYPES  OF  CENTERS  AND  MOLDS  65 

arch  masonry  during  construction,  and  of  the  roof  pressures 
which  are  transferred  to  the  center'  when  the  strutting  is 
removed  to  allow  the  masonry  to  be  placed.  The  framework 
of  the  center  is  supported  either  by  posts  resting  upon  the  floor 
of  the  excavation,  or  upon  the  invert  masonry  when  this  is 
built  first,  as  in  the  English  and  Austrian  methods,  or  it  may 
be  supported  directly  upon  the  ground  where  the  arch  masonry 
is  built  first,  as  in  the  Belgian  method  of  tunneling. 

In  describing  the  various  methods  of  tunneling  in  succeed- 
ing chapters,  the  center  construction  and  method  of  supporting 
the  center  peculiar  to  each  will  be  fully  explained,  and  only  a 
few  general  remarks  are  necessary  here.  Centers  may  be  classi- 
fied according  to  their  construction  and  composition  into  plank 
centers,  truss  centers,  and  iron  centers. 

One  of  the  most  common  forms  of  plank  centers  is  shown 
by  Fig.  42.  It  consists  of  two 
half-polygons  whose  sides  consist 
of  15  in.  X  4  ft.  planks  having 
radial  end-joints.  These  two  half- 
polygons  are  laid  one  upon  the 
other  so  that  they  break  joints,  as 

Shown    by   the    figure,   and    the    ex-     FIG.  42.— Plank  Center  for  Construct- 
ing the  Roof  Arch. 

trados  01  the  frame  is  cut  to  the 

true  curve  of  the  roof  arch.  The  planks  commonly  used  for 
making  these  centers  are  4  ins.  thick,  making  the  total  thick- 
ness of  the  center  8  ins.  Plank  centers  of  the  construction 
described  are  suitable  only  for  work  where  the  pressures  to  be 
resisted  are  small,  as  in  tunnels  through  a  fairly  firm  rock,  al- 
though there  have  been  instances  of  their  successful  use  in  soft- 
ground  tunnels. 

Where  heavy  loads  have  to  be  carried,  trussed  centers  are 
generally  employed,  the  trusses  being  composed  of  heavy  square 
beams  with  scarfed  and  tenoned  joints,  reinforced  by  iron  plates. 
Different  forms  of  trusses  are  employed  in  each  of  the  differ- 
ent methods  of  tunneling,  and  each  of  these  is  described  in  sue- 


66 


TUNNELING 


FIG.  43.  — Trussel  Center 
for  Constructing  the 
Hoof  Arch. 


ceeding  chapters  ;  but  they  are  generally  either  of  the  king-post 
or  queen-post  type,  or  so  memodification  of  them.  The  king- 
post truss  may  be  used  alone,  with  or  with- 
out the  tie-beam,  as  shown  by  Fig.  43 ;  but 
generally  a  queen-post  truss  is  made  to 
form  the  base  of  support  for  a  smaller  king 
post  truss  mounted  on  its  top.  This  arrange- 
ment gives  a  trapezoidal  form  to  the  center, 
which  approaches  closely  to  the  arch  pro- 
file. Owing  to  the  character  of  the  pres- 
sures transmitted  to  the  center,  the  usual 
tension  members  can  be  made  very  light. 

The  combined  center  and  strutting  system  devised  by  Mr. 
Rziha  has  already  been  described  in  a  previous  chapter.  In 
recent  European  tunnel  work  quite  extensive  use  has  also  been 
made  of  iron  centers  consisting  of  several  segments  of  curved 
I-beams,  connected  by  fish-plate  joints  so  as  to  form  a  semi- 
circular arch  rib.  The  ends  or  feet  of  these  I-beam  ribs  have 
bearing-plates  or  shoes  made  by  riveting  angles  to  the  I-beams. 
Centers  constructed  in  a  similar  manner,  but  made  of  sections 
of  old  railway  rail,  were  used  in  carrying  out  the  tunnel  work 
on  the  Rhine  River  Railroad  in  Germany.  The  advantages 
claimed  for  iron  centers  are  that  they  take  up  less  room,  and 
that  they  can  be  used  over  and  over  again. 

Setting  Up  Centers.  —  According  to  the  method  of  excava- 
tion followed  in  building  the  tunnel,  the  centers  for  building 
the  roof  arch  may  have  to  be  supported  by  posts  resting  on  the 
tunnel  floor ;  or  where  the  arch  is  built  first,  as  in  the  Belgian 
and  Italian  methods,  they  may  be  carried  on  blocking  resting 
on  the  unexcavated  earth  below.  Whichever  method  is  em- 
ployed, an  unyielding  support  is  essential,  and  care  must  be 
taken  that  the  centers  are  erected  and  maintained  in  a  plane 
normal  to  the  tunnel  axis.  To  prevent  deflection  and  twisting, 
the  consecutive  centers  are  usually  braced  together  by  longi- 
tudinal struts  or  by  braces  running  to  the  adjacent  strutting. 


TYPES  OF  CENTERS  AND  MOLDS  67 

Only  skilled  and  experienced  workmen  should  be  employed  in 
erecting  the  centers ;  and  they  should  work  under  the  immedi- 
ate direction  of  the  engineer,  who  must  establish  the  axis  and 
level  of  each  center  by  transit  and  level. 

Lagging.  —  By  the  lagging  is  meant  the  covering  of  narrow 
longitudinal  boards  resting  upon  the  upper  curved  chords  of  the 
centers,  and  spanning  the  opening  between  consecutive  centers. 
This  lagging  forms  the  curved  surface  or  mold  upon  which  the 
arch  masonry  is  laid.  When  the  roof  arch  is  of  ashlar  masonry 
the  strips  of  lagging  are  seldom  placed  nearer  together  than 
the  joints  of  the  consecutive  ring  stones,  but  in  brick  arches 
they  are  laid  close  together.  Besides  the  weight  of  the  arch 
masonry,  the  lagging  timbers  support  the  short  props  which 
keep  the  poling-boards  in  place  after  the  strutting  is  removed 
and  until  the  arch  masonry  is  completed. 

Striking  the  Centers.  —  The  centers  are  usually  brought  to 
the  proper  elevation  by  means  of  wooden  wedges  inserted  be- 
tween the  sill  of  the  center  and  its  support,  or  between  the 
bottom  of  the  posts  carrying  the  center  and  the  tunnel  floor. 
These  wedges  are  usually  made  of  hard  wood,  and  are  about 
6  ins.  wide  by  4  ins.  thick  by  18  ins.  long.  To  strike  the  center 
after  the  arch  masonry  is  completed,  these  wedges  are  with- 
drawn, thus  allowing  the  center  to  fall  clear  of  the  masonry. 
Usually  the  center  is  not  removed  immediately  after  striking, 
so  that  if  the  arch  masonry  fails  the  ruins  will  remain  upon  the 
center.  The  method  of  striking  the  iron  center  devised  by  Mr. 
Rziha  has  been  described  in  the  previous  chapter  on  strutting. 


68  TUNNELING 


CHAPTER   VIII. 
METHODS   OF   LINING   TUNNELS. 


TUNNELS  in  soft  soils  and  in  loose  rock,  and  rock  liable  to 
disintegration,  are  always  provided  with  a  lining  to  hold  the 
walls  and  roof  in  place.  This  lining  may  cover  the  entire 
sectional  profile  of  the  tunnel,  or  only  a  part  of  it,  and  it  may 
be  constructed  of  timber,  iron,  iron  and  masonry,  or,  more 
commonly,  of  masonry  alone. 

Timber  Lining.  —  Timber  is  seldom  employed  in  lining 
tunnels  except  as  a  temporary  expedient,  and  is  replaced  by 
masonry  as  soon  as  circumstances  will  permit.  In  the  first 
construction  of  many  American  railways,  the  necessity  for 
extreme  economy  in  construction,  and  of  getting  the  line  open 
for  traffic  as  soon  as  possible,  caused  the  engineers  to  line 
many  tunnels  with  timber,  which  was  plentiful  and  cheap. 
Except  for  their  small  cost  and  the  ease  and  rapidity  with 
which  they  can  be  constructed,  however,  these  timber  linings 
possess  few  advantages.  It  is  only  the  matter  of  a  few  years 
when  the  decay  of  the  timber  makes  it  necessary  to  rebuild 
them,  and  there  is  always  the  serious  danger  of  fire.  In 
several  instances  timber-lined  tunnels  in  America  have  been 
burned  out,  causing  serious  delays  in  traffic,  and  necessitating 
complete  reconstruction.  Usually  this  reconstruction  has  con- 
sisted in  substituting  masonry  in  place  of  the  original  timber 
lining.  In  a  succeeding  chapter  a  description  will  be  given  of 
some  of  the  methods  employed  in  replacing  timber  tunnel 
linings  with  masonry.  Various  forms  of  timber  lining  are 
employed,  of  which  Fig.  44  and  the  illustrations  in  the  chapter 


METHODS    OF   LINING    TUNNELS 


69 


discussing  the  methods  of  relining   timber-lined  tunnels  with 
masonry  are  typical  examples. 

Iron  Lining.  —  The  use  of  iron  lining  for  tunnels  was^  intro- 
duced first  on  a  large  scale  by  Mr.  Peter  William  Barlow  in 
1869,  for  the  second  tunnel  under  the  River  Thames  at 
London,  England,  and  it  has  greatly  extended  since  that  time. 
The  lining  of  the  second  Thames  tunnel  consisted  of  cylindrical 
cast-iron  rings  8  ft.  in  diameter,  the  abutting  edges  of  the 
successive  rings  being  flanged  and  provided  with  holes  for 
bolt  fastenings.  Each  ring  was  made  up  of  four  segments, 


Cross  ^Section .  Longitudinal,  Section* 

FIG.  44.  —  A  Typical  Form  of  Timber  Lining  for  Tunnels. 

three  of  which  were  longer  than  quadrants,  and  one  much 
smaller  forming  the  "  key-stone "  or  closing  piece.  These 
segments  were  connected  to  each  other  by  flanges  and  bolts. 
To  make  the  joints  tight,  strips  of  pine  or  cement  and  hemp 
yarn  were  inserted  between  the  flanges.  Since  the  construc- 
tion of  the  second  Thames  tunnel,  iron  lining  has  been  em- 
ployed for  a  great  many  submarine  tunnels  in  England, 
Continental  Europe,  and  America,  some  of  them  having  a 
section  over  28  ft.  in  diameter.  Where  circular  iron  lining  is 
employed,  the  bottom  part  of  the  section  is  leveled  up  with 
concrete  or  brick  masonry  to  carry  the  tracks,  and  the  whole 


70  TUNNELING 

interior  of  the  ring  is  covered  with  a  cement  plaster  lining- 
deep  enough  thoroughly  to  embed  the  interior  joint  flanges. 
In  the  succeeding  chapter  describing  the  methods  of  driving 
tunnels  by  shields  several  forms  of  iron  tunnel  lining  are  fully 
described. 

Iron  and  Masonry  Lining.  —  During  recent  years  a  form  of 
combined  masonry  and  iron  lining  has  been  extensively  em- 
ployed in  constructing  city  underground  railways  in  both 
Europe  and  America.  Generally  this  form  of  lining  is  built 
with  a  rectangular  section.  Two  types  of  construction  are 
employed.  In  the  first,  masonry  side  walls  carry  a  flat  roof 
of  girders  and  beams,  which  carry  a  trough  flooring  filled  with 
concrete,  or  between  which  are  sprung  concrete  or  brick  arches. 
Sometimes  the  roof  framing  consists  of  a  series  of  parallel 
I-beams  laid  transversely  across  the  tunnel,  and  in  other  cases- 
transverse  plate  girders  carry  longitudinal  I-beams.  In  the 
second  type  of  construction  the  roof  girders  are  supported  by 
columns  embedded  in  the  side  walls.  Where  the  tunnel  pro- 
vides for  two  or  four  tracks,  intermediate  column  supports  are 
in  some  cases  introduced  between  the  side  columns.  In  this 
construction  the  roofing  consists  of  concrete  filled  troughs  or  of 
concrete  or  brick  arches,  as  in  the  construction  first  described. 
Examples  of  combined  masonry  and  iron  tunnel  lining  are- 
illustrated  in  the  succeeding  chapter  on  tunneling  under  city 
streets. 

Masonry  Lining.  —  The  form  of  tunnel  lining  most  commonly 
employed  is  brick  or  stone  masonry.  Concrete  masonry  lining 
has  been  employed  in  several  tunnels  built  in  recent  years.  The 
masonry  lining  may  inclose  the  whole  section  or  only  a  part  of 
it.  The  floor  or  invert  is  the  part  most  commonly  omitted; 
but  sometimes  also  the  side  walls  and  invert  are  both  omitted, 
and  the  lining  is  confined  simply  to  an  arch  supporting  the 
roof.  The  roof  arch,  the  side  walls,  and  the  invert  compose 
the  tunnel  lining;  and  all  three  may  consist  of  stone  or  brick 
alone,  or  stone  side  walls  may  be  employed  with  brick  invert 


METHODS    OF    LINING    TUNNELS  71 

and  roof  arch.  Rubble-stone  masonry  is  usually  employed, 
except  at  the  entrances,  where  the  masonry  is  exposed  to  view. 
Here  ashlar  masonry  is  usually  used.  The  stone  selected  for 
tunnel  lining  should  be  of  a  durable  quality  which  weathers 
well.  Where  bricks  are  used  they  should  be  of  good  qual- 
ity. Owing  to  the  comparative  ease  with  which  brick  arches 
can  be  built,  they  are  generally  used  to  form  the  roof  arch,  even 
where  the  side  walls  are  of  stone  masonry.  Masonry  lining 
may  be  built  in  the  form  of  a  series  of  separate  rings,  or  in  the 
form  of  a  continuous  structure  extending  from  one  end  of  the 
tunnel  to  the  other.  The  latter  method  of  construction  pro- 
duces a  stronger  structure  ;  but  in  case  of  failure  by  crush- 
ing, the  damage  done  is  likely  to  be  more  widespread  than 
where  separate  rings  are  employed,  one  or  two  of  which 
may  fail  without  injury  to  the  others  adjacent  to  them.  The 
construction  is  also  somewhat  simpler  where  separate  rings  are 
employed,  since  no  provision  has  to  be  made  for  bonding  the 
whole  lining  into  a  continuous  structure.  Where  a  series  of 
separate  rings  is  employed,  the  length  of  each  ring  runs  from 
5  ft.  up  to  20  ft.,  it  depending  upon  the  character  of  the 
material  penetrated,  and  the  method  of  construction  employed. 
For  the  purpose  of  detailed  discussion  the  construction  of 
masonry  lining  may  be  divided  into  four  parts,  —  the  side-wall 
foundations,  the  side  walls  themselves,  the  roof  arch,  and  the 
invert. 

.  Foundations.  —  In  tunnels  through  rock  of  a  hard  and  dur- 
able character  the  foundations  for  the  side  walls  are  usually 
laid  directly  on  the  rock.  In  loose  rock,  or  rock  liable  to  dis- 
integration, this  method  of  construction  is  not  generally  a  safe 
one,  and  the  foundation  excavation  should  be  sunk  to  a  depth 
at  which  the  atmospheric  influences  cannot  affect  the  founda- 
tion bed.  In  either  case  the  foundation  masonry  is  made 
thicker  than  that  of  the  side  walls  proper,  so  as  to  distribute 
the  pressure  over  a  greater  area,  and  to  afford  more  room  for 
adjusting  the  side-wall  masonry  to  the  proper  profile.  In 


72  TUNNELING 

yielding  soils  a  special  foundation  bed  has  to  be  prepared  for 
the  foundation  masonry.  In  some  instances  it  is  found  suffi- 
cient to  lay  a  course  of  planks  upon  which  the  masonry  is  con- 
structed, but  a  more  solid  construction  is  usually  preferred. 
This  is  obtained  by  placing  a  concrete  footing  from  1  ft.  to  2 
ft.  deep  all  along  the  bottom  of  the  foundation  trench,  or  in 
some  cases  by  sinking  wells  at  intervals  along  the  trench  and 
filling  them  with  concrete,  so  as  to  form  a  series  of  supporting 
pillars. 

The  form  given  to  the  foundation  courses  and  lower 
portions  of  the  side  walls  varies.  Where 
a  large  bearing  area  is  required,  the  back 
of  the  wall  is  carried  up  vertically  as 
shown  by  the  line  AB,  Fig.  45,  otherwise 
the  rear  face  of  the  wall  follows  the  line  of 
excavation  A  C.  For  similar  reasons  the  front 

_  face  of  the  wall  may  be  made  vertical,  as  at 

B          C         H    6        jvj,     or  inclined     as   at  FSm       The   line 


FlG.    45.  —  Diagram 

showing     Forms     indicates    the     shelf    construction    designed 

Adopted  for  Side-  ,  •.  />  /.      ,  -,  -i     . 

waii  Foundations.  *°  support  the  feet  of  the  posts  used  to 
carry  the  arch  centers  during  the  construc- 
tion of  the  roof  arch. 

Side  Walls.  —  The  construction  of  the  side  walls  above  the 
foundation  courses  is  carried  out  as  any  similar  piece  of 
masonry  elsewhere  would  be  built.  To  direct  the  work  and 
insure  that  the  inner  faces  of  the  walls  follow  accurately  the 
curve  of  the  chosen  profile,  leading  frames  previously  described 
are  employed. 

Roof  Arch.  —  For  the  construction  of  the  roof  arch,  the 
centers  previously  described  are  employed.  Beginning  at  the 
edges  of  the  center  on  each  side,  the  masonry  is  carried  up  a 
course  at  a  time,  care  being  taken  to  have  it  progress  at  the 
same  rate  on  both  sides,  so  that  the  load  brought  onto  the 
centering  is  symmetrical.  As  soon  as  the  centers  are  erected, 
the  roof  strutting  is  removed,  and  replaced  by  short  props 


METHODS    OF    LINING    TUNNELS  73 

which  rest  on  the  lagging  of  the  centers  and  support  the  poling- 
boards.  These  props  are  removed  in  succession  as  the  arch 
masonry  rises  along  the  curve  of  the  center,  and  the  space 
between  the  top  of  the  arch  masonry  and  the  ceiling  ftf  the 
excavation  is  filled  with  small  stones  packed  closely.  The  key- 
stone section  of  the  arch  is  built  last,  by  inserting  the  stones  or 
bricks  from  the  front  edge  of  the  arch  ring,  there  being  no 
room  to  set  them  in  from  the  top,  as  is  the  practice  in  ordinary 
open-arch  construction.  The  keying  of  the  arch  is  an  espe- 
cially difficult  operation,  and  only  experienced  men  skilled  in  the 
work  should  be  employed  to  perform  it.  The  task  becomes 
one  of  unusual  difficulty  when  it  becomes  necessary  to  join  the 
arches  coming  from  opposite  directions. 

Invert.  —  In  all  but  one  or  two  methods  of  tunneling,  the 
invert  is  the  last  portion  of  the  lining  to  be  built.  In  the 
English  method  of  tunneling,  the  invert  is  the  first  portion  of 
the  lining  to  be  built,  and  the  same  practice  is  sometimes  neces- 
sary in  soft  soils  where  there  is  danger  of  the  bottoms  of  the 
side  walls  being  squeezed  together  by  the  lateral  pressures 
unless  the  invert  masonry  is  in  place  to  hold  them  apart.  The 
ground  molds  previously  described  are  employed  to  direct  the 
construction  of  the  invert  masonry. 

General  Observations.  —  In  describing  the  construction  of  the 
roof  arch,  mention  was  made  of  the  stone  filling  employed 
between  the  back  of  the  masonry  ring  and  the  ceiling  of  the 
excavation.  The  spaces  behind  the  side  walls  are  filled  in  a 
similar  manner.  The  object  of  this  stone  filling,  which  should 
be  closely  packed,  is  to  distribute  the  vertical  and  lateral  press- 
ures in  the  walls  of  the  excavation  uniformly  over  the  lining 
masonry.  As  the  masonry  work  progresses,  the  strutting 
employed  previously  to  support  the  walls  of  the  excavation  has 
to  be  removed.  This  work  requires  care  to  prevent  accident, 
and  should  be  placed  in  charge  of  experienced  mechanics  who 
are  familiar  with  its  construction,  and  can  remove  it  with  the 
least  damage  to  the  timbers,  so  that  they  may  be  used  again, 


74  TUNNELING 

and  without  endangering  the  fall  of  the  roof  or  the  caving  of 
the  sides  by  removing  too  great  a  portion  of  the  timbers  at  one 
time. 

Thickness  of  Lining  Masonry.  —  It  is  obvious,  of  course,  that 
the  masonry  lining  must  be  thick  enough  to  support  the  press- 
ure of  the  earth  which  it  sustains ;  but,  as  it  is  impossible  to 
estimate  these  pressures  at  all  accurately,  it  is  difficult  to  say 
definitely  just  what  thickness  is  required  in  any  individual  case. 
Rankine  gives  the  following  formulas  for  determining  the 
depths  of  keystone  required  in  different  soils  : 
For  firm  soils, 


and  for  soft  soils, 

d  = 

where  d  =  the  depth  of  the  crown  in  feet,  r  =  the  rise  of  the 
arch  in  feet,  and  s  =  the  span  of  the  arch  in  feet.  Other 
writers,  among  them  Professor  Curioni,  attempt  to  give  rational 
methods  for  calculating  the  thickness  of  tunnel  lining ;  but  they 
are  all  open  to  objection  because  of  the  amount  of  hypothesis 
required  concerning  pressures  which  are  of  necessity  indetermi- 
nate. Therefore,  to  avoid  tedious  and  uncertain  calculations, 
the  engineer  adopts  dimensions  which  experience  has  proven  to 
be  ample  under  similar  conditions  in  the  past.  Thus  we  have 
all  gradations  in  thickness,  from  hard-rock  tunnels  requiring 
no  lining,  and  tunnels  through  rocks  which  simply  require  a 
thin  shell  to  protect  them  from  the  atmosphere,  to  soft-ground 
tunnels  where  a  masonry  lining  3  ft.  or  more  in  thickness  is 
employed.  Table  II.  shows  the  thickness  of  masonry  lining 
used  in  tunnels  through  soft  soils  of  various  kinds. 

The  thickness  of  the  masonry  lining  is  seldom  uniform  at 
all  points,  as  is  indicated  by  Table  II.  Figs.  46  and  47  show 
common  methods  of  varying  the  thickness  of  lining  at  different 
points,  and  are  self-explanatory. 


METHODS    OF   LINING    TUNNELS  75 

Side  Tunnels. — When  tunnels  are  excavated  by  shafts  located 
at  one  side  of  the  center  line,  short  side  tunnels  or  galleries  are 
built  to  connect  the  bottoms  of  the  shafts  with  the  tunnel 

^h 

proper.  These  side  tunnels  are  usually  from  30.  ft  to  40  ft. 
long,  and  are  generally  made  from  12  ft.  to  14  ft.  high,  and 
about  10  ft.  wide.  The  excavation,  strutting,  and  lining  of 
these  side  tunnels  are  carried  on  exactly  as  they  are  in  the 
main  tunnel,  with  such  exceptions  as  these  short  lengths 
make  possible.  Table  III.  gives  the  thickness  of  lining  used 
for  side  tunnels,  the  figures  being  taken  from  European 
practice. 


FIGS.  46  and  47. —Transverse  Sections  of  Tunnels  Showing  Methods  of  Increasing  the 
Thickness  of  the  Lining  at  Different  Points. 

Culverts,  —  The  purpose  of  culverts  in  tunnels  is  to  collect 
the  water  which  seeps  into  the  tunnel  from  the  walls  and  shafts. 
The  culvert  is  usually  located  along  the  center  line  of  the 
tunnel  at  the  bottom.  In  soft-ground  tunnels  it  is  built  of 
masonry,  and  forms  a  part  of  the  invert,  but  in  rock  tunnels  it 
is  the  common  practice  to  cut  a  channel  in  the  rock  floor  of  the 
excavation.  Both  box  and  arch  sections  are  employed  for 
culverts.  The  dimensions  of  the  section  vary,  of  course,  with 
the  amount  of  water  which  has  to  be  carried  away.  The  fol- 
lowing are  the  dimensions  commonly  employed : 


76 


KIND  OF  CTTLVEBT. 

HEIGHT  IN 
FEET. 

WIDTH  IN 
FEET. 

THICKNESS  OF 
WALLS 
IN  FEET. 

THICKNESS 
OF  COY  EKING 
IN  FEET. 

Box  culvert   .... 
Arch  culvert  .... 

1  to  1.5 
1  to  1.5 

Itol  5 
1  to  1.5 

0.8  to  1.2 
0.8  to  1.2 

0.3 
0.4 

It  should  be  understood  that  the  dimensions  given  in  the 
table  are  those  for  ordinary  conditions  of  leakage ;  where  larger 
quantities  of  water  are  met  with,  the  size  of  the  culverts  has, 
of  course,  to  be  enlarged.  To  permit  the  water  to  enter  the 
culvert,  openings  are  provided  at  intervals  along  its  side ;  and 
these  openings  are  usually  provided  with  screens  of  loose  stones 
which  check  the  current,  and  cause  the  suspended  material  to 


1., 


FIG.  48.  — Refuge  Niche  in  St.  Gothard  Tunnel. 

be  deposited  before  it  enters  the  culvert.  In  cases  where 
springs  are  encountered  in  excavating  the  tunnel,  it  is  necessary 
to  make  special  provisions  for  confining  their  outflow  and  con- 
ducting it  to  the  culvert.  In  all  cases  the  culverts  should  be 
provided  with  catch  basins  at  intervals  of  from  150  ft.  to  300 
ft.,  in  which  such  suspended  matter  as  enters  the  culverts  is 
deposited,  and  removed  through  covered  openings  over  each 
basin.  At  the  ends  of  the  tunnel  the  culvert  is  usually  divided 
into  two  branches,  one  running  to  the  drain  on  each  side  of  the 
track. 

Niches.  —  In  short  tunnels  niches  are  employed  simply  as 
places  of  refuge  for  trackmen  and  others  during  the  passing  of 
trains,  and  are  of  small  size.  In  long  tunnels  they  are  made 


METHODS    OF   LINING   TUNNELS 


77 


larger,  and  are  also  employed  as  places  for  storing  small  tools 
and  supplies  employed  in  the  maintenance  of  the  tunnel. 
Niches  are  simply  arched  recesses  built  into  the  sides  of  the 
tunnel,  and  lined  with  masonry ;  Fig.  48  shows  this  construc- 
tion quite  clearly.  Small  refuge  niches  are  usually  built  from 
6  ft.  to  9  ft.  high,  from  3  ft.  to  6  ft.  wide,  and  from  2  ft. 
to  3  ft.  deep.  Large  niches  designed  for  storing  tools  and 
supplies  ai-e  made  from  10  ft.  to  12  ft.  high,  from  8  ft.  to  10  ft. 
wide,  and  from  18  ft  to  24  ft.  deep,  and  are  provided  with 


FIG.  49.  —  East  Portal  of  Hoosac  Tunnel. 

doors.  Refuge  niches  are  usually  spaced  from  60  ft.  to  100  ft. 
apart,  while  the  larger  storage  niches  may  be  located  as  far  as 
3000  ft.  apart.  The  niche  construction  shown  by  Fig.  47  is 
that  employed  on  the  St.  Gothard  tunnel. 

Entrances.  —  The  entrances,  or  portals,  of  tunnels  usually 
consist  of  more  or  less  elaborate  masonry  structures,  depending 
upon  the  nature  of  the  material  penetrated.  In  soft-ground 
tunnels  extensive  wing  walls  are  often  required  to  support  the 
earth  above  and  at  the  sides  of  the  entrance ;  while  in  tunnels 


78 


TUNNELING 


through  rock,  only  a  masonry  portal  is  required,  to  give  a  finish 
to  the  work.  Often  the  engineer  indulges  himself  in  an  elabo- 
rate architectural  design  for  the  portal  masonry.  There  is 
danger  of  carrying  such  designs  too  far  for  good  taste  unless 
care  is  employed ;  and  011  this  matter  the  writer  can  do  no  better 
than  to  quote  the  remarks  of  the  late  Mr.  Frederick  W.  Simms 
in  his  well-known  "  Practical  Tunneling  "  : 

"  The  designs  for  such  constructions  should  be  massive  to  be  suitable  as 
approaches  to  works  presenting  the  appearance  of  gloom,  solidity,  and  strength. 
A  light  and  highly  decorated  structure,  however  elegant  and  well  adapted  for 
other  purposes,  would  be  very  unsuitable  in  such  a  situation ;  it  is  plainness 
combined  with  boldness,  and  massiveness  without  heaviness1  that  in  a  tunnel 
•entrance  constitutes  elegance,  and,  at  the  same  time,  is  the  most  economical." 

Fig.  49  is  an  engraving  from  a  photograph  of  the  east  portal 
of  the  Hoosac  tunnel,  which  is  an  especially  good  design. 


TABLE   II. 
Showing  Thickness  of  Masonry  Lining  for  Tunnels  through  Soft  Ground. 


CHARACTER  OF  MATERIAL. 

KEYSTONE. 

SPRINGERS. 

INVERT. 

Laminated  clay,  first  variety  .     . 
Laminated  clay,  second  variety   . 
Laminated  clay,  third  variety  . 
Quicksand                                  . 

Ft. 

2.15  to  3 
3       to  4.5 
4.5    to  6.5 
2       to  3.28 

Ft. 

2.75  to  3.5 
3.5    to  5.5 
5.5    to  8.1 
2       to  4  1 

Ft. 

1.6    to  2.5 
2.5    to  4 
4       to  4.  5 
1  33  to  2  5 

TABLE   III. 

Showing  Thickness  of  Masonry  Lining  for  Side  Tunnels  through 
Soft  Ground. 


CHARACTER  OF  MATERIAL. 

KEYSTONE. 

SPRINGERS. 

INVERT. 

Laminated  clay,  first  variety  .     . 
Laminated  clay,  second  variety   . 
Laminated  clay,  third  variety 
Quicksand   .... 

Ft. 

1.6  to  2.3 
2.3  to  3 

3     to  4 
1  6  to  2  5 

Ft. 

1.8  to  3 
3     to  4.1 
4.1  to  5 
1  3  to  2 

Ft. 

1.5  to  2 
2     to  2  6 
2.6  to  3.29 
1  3  to  2 

TUNNELS   THROUGH   HARD   ROCK  79 


CHAPTER  IX. 

TUNNELS     THROUGH     HARD    ROCK ;    GENERAL 
DISCUSSION;      EXCAVATION      BY     DRIFTS. 
MONT    CENIS    TUNNEL. 


THE  present  high  development  of  labor-saving  machinery 
for  excavating  rock  makes  this  material  one  of  the  safest  and 
easiest  to  tunnel  of  any  with  which  the  engineer  ordinarily  has 
to  deal.  To  operate  this  machinery  requires,  however,  the 
development  of  a  large  amount  of  power,  its  transmission  to 
considerable  distances,  and,  finally,  its  economical  application 
to  the  excavating  tools.  The  standard  rock  excavating  ma- 
chine is  the  power  drill,  which  requires  either  air  or  hydraulic 
pressure  for  its  operation  according  to  the  special  type  em- 
ployed. Under  present  conditions,  therefore,  the  engineer  is 
limited  either  to  air  or  water  under  compression  for  the  trans- 
mission of  his  power.  Steam-power  may  be  employed  directly 
to  operate  percussion  rock  drills ;  but  owing  to  the  neat  and 
humidity  which  it  generates  in  the  confined  space  where  the 
drills  work,  and  because  of  other  reasons,  it  is  seldom  employed 
directly.  Electric  transmission,  which  offers  so  many  advan- 
tages to  the  tunnel  builder,  in  most  respects  is  largely  excluded 
from  use  by  the  failure  which  has  so  far  followed  all  attempts 
to  apply  it  to  the  operation  of  rock  drills.  As  matters  stand, 
therefore,  the  tunnel  engineer  is  practically  limited  to  steam 
and  falling  water  for  the  generation  of  power,  and  to  com- 
pressed air  and  hydraulic  pressure  for  its  transmission. 

Whether  the  engineer  should  adopt  water-power  or  steam  to 
generate  the  power  required  for  his  excavating  machinery  de- 
pends upon  their  relative  availability,  cost,  and  suitability  to  the 


80  TUNNELING 

conditions  of  work  in  each  particular  case.  Where  fuel  is  plen- 
tiful and  cheap,  and  where  water-power  is  not  available  at  a 
comparatively  reasonable  cost,  steam-power  will  nearly  always 
prove  the  more  economical ;  where,  however,  the  reverse  con- 
ditions exist,  which  is  usually  the  case  in  a  mountainous 
country  far  from  the  coal  regions,  and  inadequately  supplied 
with  transportation  facilities,  but  rich  in  mountain  torrents,, 
water-power  will  generally  be  the  more  economical.  In  a  suc- 
ceeding chapter  the  power  generating  and  transmission  plants 
for  a  number  of  rock  tunnels  are  described,  and  here  only  a 
general  consideration  of  the  subject  will  be  presented. 

Steam-Power  Plant.  —  A  steam-power  plant  for  tunnel  work 
should  be  much  the  same  as  a  similar  plant  elsewhere,  except 
that  in  designing  it  the  temporary  character  of  its  work  must 
be  taken  into  consideration.  This  circumstance  of  its  tempo- 
rary employment  prompts  the  omission  of  all  construction 
except  that  necessary  to  the  economical  working  of  the  plant 
during  the  period  when  its  operation  is  required.  The  power- 
house, the  foundations  for  the  machinery,  and  the  general  con- 
struction and  arrangement,  should  be  the  least  expensive  which 
will  satisfy  the  requirements  of  economical  and  safe  operation 
for  the  time  required.  It  will  often  be  found  more  economical 
as  a  whole  to  operate  the  machinery  with  some  loss  of  economy 
during  the  short  time  that  it  is  in  use  than  to  go  to  much 
greater  expense  to  secure  better  economy  from  the  machinery 
by  design  and  construction,  which  will  be  of  no  further  use 
after  the  tunnel  is  completed.  The  longer  the  plant  is  to  be 
required,  the  nearer  the  construction  may  economically  approach 
that  of  a  permanent  plant.  As  regards  the  machinery  itself, 
whose  further  usefulness  is  not  limited  by  the  duration  of  any 
single  piece  of  work,  true  economy  always  dictates  the  purchase 
of  the  best  quality.  Speaking  in  a  general  way,  a  steam-power 
plant  for  tunnel  work  comprises  a  boiler  plant,  a  plant  of  air 
compressors  with  their  receivers,  and  an  electric  light  dynamo. 
When  hydraulic  transmission  of  power  is  employed,  the  air 


TUNNELS   THROUGH    HAUD   ROCK  81 

compressors  are  replaced  by  high-pressure  pumps ;  and  when 
electric  hauling  is  employed,  one  or  more  dynamos  may  be  re- 
quired to  generate  electricity  for  power  purposes,  as  well  as  for 
lighting.  In  addition  to  the  power  generating  machines  proper, 
there  must  be  the  necessary  piping  and  wiring  for  transmitting 
this  power,  and,  of  course,  the  equipment  of  drills  and  other 
machines  for  doing  the  actual  excavating,  hauling,  etc. 

Reservoirs.  —  When  water-power  is  employed,  a  reservoir 
has  to  be  formed  by  damming  some  near-by  mountain  stream  at 
a  point  as  high  a«  practicable  above  the  tunnel.  The  provision 
of  a  reservoir,  instead  of  drawing  the  water  directly  from  the 
stream,  serves  two  important  purposes.  It  insures  a  continuous 
supply  and  constant  head  of  water  in  case  of  drought,  and  also 
permits  the  water  to  deposit  its  sediment  before  it  is  delivered 
to  the  turbines.  The  construction  of  these  reservoirs  may  be 
of  a  temporary  character,  or  they  may  be  made  permanent 
structures,  and  utilized  after  construction  is  completed  to  sup- 
ply power  for  ventilation  and  other  necessary  purposes.  In  the 
first  case  they  are  usually  destroyed  after  construction  is  fin- 
ished. In  either  case,  it  is  almost  unnecessary  to  say,  they 
should  be  built  amply  safe  and  strong  according  to  good  engi- 
neering practice  in  such  works,  for  the  duration  of  time  which 
they  are  expected  to  exist. 

Canals  and  Pipe  Lines.  —  For  conveying  the  water  from  the 
reservoirs  to  the  turbines,  canals  or  pipe  lines  are  employed. 
The  latter  form  of  conduit  is  generally  preferable,  it  being 
both  less  expensive  and  more  easily  constructed  than  the 
former.  It  is  advisable  also  to  have  duplicate  lines  of  pipe  to 
reduce  the  possibility  of  delay  by  accident  or  while  necessary 
repairs  are  being  made  to  one  of  the  pipes.  The  pipe  lines 
terminate  in  a  penstock  leading  into  the  turbine  chamber,  and 
provided  with  the  necessary  valves  for  controlling  the  admis- 
sion of  water  to  the  turbines. 

Turbines.  —  There  are  numerous  forms  of  turbines  on  the 
market,  but  they  may  all  be  classed  either  as  impulse  turbines 


82  TUNNELING 

or  as  reaction  turbines.  Impulse  turbines  are  those  in  which 
the  whole  available  energy  of  the  water  is  converted  into 
kinetic  energy  before  the  water  acts  on  the  moving  part  of  the 
turbine.  Reaction  turbines  are  those  in  which  only  a  part  of 
the  available  energy  of  the  water  is  converted  into  kinetic 
energy  before  the  water  acts  on  the  moving  vanes.  Impulse 
turbines  give  efficient  results  with  any  head  and  quantity  of 
water,  but  they  give  better  results  when  the  quantity  of  water 
varies  and  the  head  remains  constant.  Reaction  turbines,  on 
the  contrary,  give  better  results  when  the  quantity  of  water 
remains  constant  and  the  head  varies.  These  observations 
indicate  in  a  general  way  the  form  of  turbine  which  will  best 
meet  the  particular  conditions  in  each  case.  The  number  of 
turbines  required,  and  their  dimensions,  will  be  determined  in 
each  case  by  the  number  of  horse-power  required  and  the 
quantity  of  water  available.  The  power  of  the  turbines  is 
transmitted  to  the  air  compressors  or  pumps  by  shafting  and 
gearing. 

Air  Compressors.  —  An  air  compressor  is  a  machine  —  usually 
driven  by  steam,  although  any  other  power  may  be  used  —  by 
which  air  is  compressed  into  a  receiver  from  which  it  may  be 
piped  for  use.  For  a  detailed  description  of  the  various  forms 
of  air  compressors  the  reader  should  consult  the  catalogues  of 
the  several  makers  and  the  various  textbooks  relating  to  air 
compression  and  compressed  air.  Air  compressors,  like  other 
machines,  suffer  a  loss  of  power  by  friction.  The  greatest  loss 
of  power,  however,  results  from  the  heat  of  compression. 
When  air  is  compressed,  it  is  heated,  and  its  relative  volume 
is  increased.  Therefore,  a  cubic  foot  of  hot  air  in  the  com- 
pressor cylinder,  at  say,  60  Ibs.  pressure,  does  not  make  a  cubic 
foot  of  air  at  60  Ibs.  pressure  after  cooling  in  the  receiver. 
In  other  words,  assuming  pressure  to  be  constant,  a  loss  of 
volume  results  due  to  the  extraction  of  the  heat  of  compression 
after  the  air  leaves  the  compressor  cylinder.  To  reduce  the 
amount  of  this  loss,  air  compressors  are  designed  with  means 


TUNNELS   THROUGH   HARD   ROCK  83 

to  extract  the  heat  from  the  air  before  it  leaves  the  com- 
pressor cylinder.  Air  compressors  may  first  be  divided  into 
two  classes,  according  to  the  means  employed  for  cooling  the 
air,  as  follows:  (1)  Wet  compressors,  and  (2)  dry  comprtss- 
ors.  A  wet  compressor  is  one  which  introduces  water  directly 
into  the  cylinder  during  compression,  and  a  dry  compressor  is 
one  which  admits  no  water  to  the  air  during  compression. 
Wet  compressors  may  be  subdivided  into  two  classes :  (1) 
Those  which  inject  water  in  the  form  of  spray  into  the  cylinder 
during  compression,  and  (2)  those  which  use  a  water  piston 
for  forcing  the  air  into  confinement. 

The  following  brief  discussion  of  these  various  types  of 
compressors  is  based  on  the  concise  practical  discussion  of 
Mr.  W.  L.  Saunders,  M.  Am.  Soc.  C.  E.,  in  "  Compressed  Air 
Production."  The  highest  isothermal  results  are  obtained  by 
the  injection  of  water  into  the  cylinders,  since  it  is  plain  that 
the  injection  of  cold  water,  in  the  shape  of  a  finely  divided 
spray,  directly  into  the  air  during  compression  will  lower  the 
temperature  to  a  greater  degree  than  simply  to  surround  the 
cylinder  and  parts  by  water  jackets  which  is  the  means  of  cool- 
ing adopted  with  dry  compressors.  A  serious  obstacle  to  water 
injection,  and  that  which  condemns  this  type  of  compressor,  is 
the  influence  of  the  injected  water  upon  the  air  cylinder  and 
parts.  Even  when  pure  water  is  used,  the  cylinders  wear  to 
such  an  extent  as  to  produce  leakage  and  to  require  reboring. 
The  limitation  to  the  speed  of  a  compressor  is  also  an  important 
objection.  The  chief  claim  for  the  water  piston  compressor  is 
that  its  piston  is  also  its  cooling  device,  and  that  the  heat  of 
compression  is  absorbed  by  the  water.  Water  is  so  poor  a 
conductor  of  heat,  however,  that  without  the  addition  of  sprays 
it  is  safe  to  say  that  this  compressor  has  scarcely  any  cooling 
advantages  at  all  so  far  as  the  cooling  of  the  air  during  com- 
pression is  concerned.  The  water  piston  compressor  operates 
at  slow  speed  and  is  expensive.  Its  only  advantage  is  that  it 
has  no  dead  spaces.  In  the  dry  compressor  a  sacrifice  is  made 


84  TUNNELING 

in  the  efficiency  of  the  cooling  device  to  obtain  low  first  cost,, 
economy  in  space,  light  weight,  higher  speed,  greater  durability, 
and  greater  general  availability. 

Air  compressors  are  also  distinguished  as  double  acting  and 
simple  acting.  They  are  simple  acting  when  the  cylinder  is 
arranged  to  take  in  air  at  one  stroke  and  force  it  out  at  the 
next,  and  they  are  double  acting  when  they  take  in  and  force 
out  air  at  each  stroke.  In  form  compressors  may  be  simple  or 
duplex.  They  are  simple  when  they  have  but  one  cylinder, 
and  duplex  when  they  have  two  cylinders.  A  straight  line  or 
direct  acting  compressor  is  one  in  which  the  steam  and  air 
cylinders  are  set  tandem.  An  indirect  acting  compressor  is 
one  in  which  the  power  is  applied  indirectly  to  the  piston  rod 
of  the  air  cylinder  through  the  medium  of  a  crank.  Mr.  W.  L, 
Saunders  writes  in  regard  to  direct  and  indirect  compression 
as  follows :  — 

"  The  experience  of  American  manufacturers,  which  has  been  more  exten- 
sive than  that  of  others,  has  proved  the  value  of  direct  compression  as  distin- 
guished from  indirect.  By  direct  compression  is  meant  the  application  of 
power  to  resistance  through  a  single  straight  rod.  The  steam  and  air  cylinders- 
are  placed  tandem.  Such  machines  naturally  show  a  low  friction  loss  because 
of  the  direct  application  of  power  to  resistance.  This  friction  loss  has  been 
recorded  as  low  as  5  %,  while  the  best  practice  is  about  10  %  with  the  type  which 
conveys  the  power  through  the  angle  of  a  crank  shaft  to  a  cylinder  connected 
to  the  shaft  through  an  additional  rod." 

Receivers.  — Compressed  air  is  stored  in  receivers  which  are 
simply  iron  tanks  capable  of  withstanding  a  high  internal 
pressure.  The  puipose  of  these  tanks  is  to  provide  a  reservoir 
of  compressed  air,  and  also  to  allow  the  air  to  deposit  its 
moisture.  From  the  receivers  the  air  is  conveyed  to  the  work- 
ings through  iron  pipes,  which  decrease  gradually  in  diameter 
from  the  receivers  to  the  front. 

Rock  Drills.  —  The  various  forms  of  rock  drills  used  in  tun- 
neling have  been  described  in  Chapter  III.,  and  need  not  be 
considered  in  detail  here  except  to  say  that  American  engi- 


TUNNELS  THROUGH  HARD  ROCK     \       85 

neers  usually  employ  percussion  drills,  while  European 
engineers  also  use  rotary  drills  extensively.  A  comparison 
between  these,  two  types  of  drills  was  made  in  excavating  the 
Aarlberg  tunnel  in  Austria,  where  the  Brandt  hydraulic 
rotary  drill  was  used  at  one  end,  and  the  Ferroux  percussion 
drill  was  used  at  the  other  end.  The  rock  was  a  mica-schist. 
The  average  monthly  progress  was  412  ft.,  with  a  maximum 
of  646  ft.,  with  the  rotary  drills,  and  an  average  of  454  ft.  with 
the  percussion  drill. 

Excavation.  —  Since  considerable  time  is  required  to  get  the 
power  plant  established,  the  excavation  of  rock  tunnels  is  often 
begun  by  hand,  but  hand  work  is  usually  continued  for  no 
longer  a  period  than  is  necessary  to  get  the  power  plant  in 
operation.  Generally  speaking,  the  greatest  difficulty  is 
encountered  in  excavating  the  advanced  drift  or  heading. 
Based  on  the  mode  of  blasting  employed,  there  are  two  methods 
of  driving  the  advanced  gallery,  known  as  the  circular  cut 
and  the  center  cut  methods.  In  the  first  method  a  set  of  holes 
is  first  drilled  near  the  center  of  the  front  in  such  a  manner  that 
they  inclose  a  cone  of  rock ;  the  holes,  starting  at  the  perimeter 
of  the  base  of  the  cone,  converge  toward  a  junction  at  its 
apex.  Seldom  more  than  four  to  six  holes  are  comprised  in 
this  first  set.  Around  these  first  holes  are  driven  a  ring 
of  holes  which  inclose  a  cylinder  of  rock,  and  if  necessary 
succeeding  rings  of  holes  are  driven  outside  of  the  first  ring. 
These  holes  are  blasted  in  the  order  in  which  they  are  driven, 
the  first  set  taking  out  a  cone  of  rock,  the  second  set  enlarging 
this  cone  to  a  cylinder,  and  the  other  sets  enlarging  this 
cylinder.  These  holes  are  seldom  driven  deeper  than  4  or  5 
ft.  In  the  center-cut  method,  which  is  the  one  commonly 
employed  in  America,  the  holes  are  arranged  in  vertical  rows, 
and  are  driven  from  15  ft.  to  20  ft.  deep.  Figs.  50  to  53 
inclusive  show  the  arrangement  of  the  holes  and  the  method 
°f  blasting  them.  The  two  center  rows  of  holes  converge 
toward  each  other  so  as  to  take  out  a  wedge  of  rock,  but  the 


86 


TUNNELING 


others  are  bored  "  straight "  or  parallel  with  the  vertical  plane 
of  the  tunnel. 

The  width  of  the  advanced  gallery  or  heading  depends 
upon  the  quality  of  the  rock.  In  hard  rock  American  engi- 
neers give  it  the  full  width  of  the  tunnel  section;  but  this 
cannot  be  done  in  loose  or  fissured  rock,  which  has  to  be  sup- 
ported, the  headings  here  being  usually  made  about  8  x  5  ft. 
The  wider  heading  is  always  preferable,  where  it  is  possible, 


PlGS.  60  to  53.  —  Sketches  Illustrating  American  Center-Cut  Method  of  Blasting  Tunnels. 

since  more  room  is  available  for  removing  the  rock,  and  deeper 
holes  can  be  bored  and  blasted. 

With  the  preceding  general  discussion  of  tunneling  through 
rock  we  may  proceed  to  a  detailed  consideration  of  the  con- 
struction of  typical  examples  of  rock  tunnels.  For  this  pur- 
pose the  Mont  Cenis  and  Simplon  tunnels  are  selected  as 
examples  of  rock  tunnels  driven  by  a  drift,  and  the  St.  Goth- 
ard  and  Busk  tunnels  as  examples  of  rock  tunnels  driven  by 
headings. 


TUNNELS    THROUGH    HARD   ROCK 


87 


EXCAVATION  BY   DRIFTS:    MONT  CENIS  TUNNEL. 

General  Description.  —  The  method  of  tunneling  tfirough 
hard  rock  by  drifts  is  preferred  by  European  engineers.  Both 
the  Mont  Cenis  tunnel  built  in  1857-70,  and  the  great  Simplon 
tunnel  now  under  construction,  are  examples  of  tunneling  by 
drifts.  In  this  method  the  sequence  of  excavation  is  shown 
diagrammatically  by  Fig.  54.  As  soon  as  the  top  portion  of 
the  section  has  been  opened,  the  roof  arch  is  built  with  its  feet 
resting  on  the  tops  of  parts  No.  4.  These  parts  are  removed 
by  breaking  down  the  outer  portion 
between  the  sides  of  part  No.  1  and 
the  lines  a  b  and  a1  bl  first,  and 
then  by  driving  transverse  cuts 
through  to  the  sides  of  the  section 
at  intervals,  and  filling  them  with 
the  masonry  of  the  side  walls. 
These  short  sections  or  pillars  of 
masonry  serve  to  carry  the  arch 
while  the  rock  between  them  is 
being  excavated  and  the  remainder 
of  the  side  walls  built.  In  hard 
rock  the  successive  parts  Xos.  1  to 
4  are  driven  several  hundred  feet  in  advance  of  each  other. 

The  drift  is  usually  strutted  by  means  of  side  posts  carrying 
a  cap-piece  placed  at  intervals,  and  having  a  ceiling  of  longi- 
tudinal planks  resting  on  the  successive  caps.  In  hard  rock 
the  roof  of  the  section  does  not,  as  a  rule,  require  regular 
strutting,  occasional  supports  being  placed  at  intervals  to  pre- 
vent the  fail  of  isolated  fragments.  When  the  rock  is  disinte- 
grated or  full  of  seams,  a  regular  strutting  may  be  necessary, 
and  this  may  be  either  longitudinal  or  polygonal  in  type. 
When  longitudinal  strutting  is  employed,  a  sill  is  laid  across 
the  roof  of  the  drift,  and  upon  this  are  set  up  two  struts  con- 


a  cr 

FIG.  54.  — Diagram  Showing  Se- 
quence of  Excavations  in  Drift 
Method  of  Tunneling  Kock. 


88  TUNNELING 

verging  toward  the  top  and  supporting  a  cap-piece  close  to 
the  roof.  On  this  cap-piece  are  placed  the  first  longitudinal 
crown  bars  carrying  transverse  poling-boards.  Additional 
props  standing  on  the  sill  and  radiating  outward  are  inserted 
as  parts  No.  3  are  excavated.  These  radial  props  carry 
longitudinal  bars  which  in  turn  support  transverse  poling- 
boards.  When  polygonal  strutting  is  used,  it  may  have  the 
construction  described  below  as  being  employed  in  the  Mont 
Cenis  tunnel,  or  may  take  the  form  of  three  or  five  segment 
arches  of  heavy  timbers. 

The  roof  arch,  usually  of  brick  masonry,  is  built  before  the 
side  walls,  which  are  generally  of  rubble  masonry,  with  its  feet 
supported  temporarily  by  the  unexcavated  rock  below.  Plank 
centers  are  usually  employed,  since  the  pressures  they  carry  are 
usually  limited  to  the  weight  of  the  masonry.  The  method 
of  underpinning  the  roof  arch  with  the  side  walls  is  that  pecu- 
liar to  the  Belgian  method  of  tunneling.  The  drain  is  usually 
constructed  of  brick  masonry,  and  may  be  located  at  the  center 
or  at  one  side  of  the  tunnel  floor. 

Tunnels  excavated  by  drifts  enable  simple  means  of  hauling 
to  be  employed,  and  this  is  one  of  the  reasons  why  the  method 
finds  so  much  favor  with  European  engineers.  The  tracks 
are  laid  along  the  floor  of  the  drift,  and  carry  all  the  spoil 
from  parts  Nos.  2,  3,  and  4,  as  well  as  from  the  front  of  the 
drift  itself.  As  fast  as  the  full  section  is  completed,  this  single 
track  in  the  drift  is  replaced  by  two  tracks  running  close  to 
the  sides  of  the  tunnel,  or  by  a  broad-gauge  track  with  a  third 
rail. 

Mont  Cenis  Tunnel.  —  The  Mont  Cenis  tunnel  -was  the  first 
of  the  great  Alpine  tunnels  to  be  built.  It  is  7.9  miles  long, 
and  connects  France  and  Italy  by  a  double-track  railway.  Con- 
struction was  begun  in  1857,  and  the  tunnel  was  opened  for 
traffic  in  1872. 

Material  Penetrated.  —  The  material  penetrated  by  the  ex- 
cavation consisted  chiefly  of  limestone,  calcareous  schist,  gneiss, 


TUNNELS    THROUGH    HARD    HOCK 


89 


and  schistose  sandstone.  The  stratification  of  the  rock  was 
nearly  perpendicular  to  the  axis  of  the  tunnel,  except  at  a  few 
points  where  the  strata  intersected  the  axis  at  angles  varying 
from  35°  to  60°. 

Excavation.  —  The  tunnel  was  driven  exclusively  from  the 
-ends  by  means  of  a  drift.     The  diagram,  Fig.  55,  shows  the 
order  of  the  excavation,  which  began  by  the  drift  No.  1,  whose 
dimensions  were  9.5  x  8.5  ft.,  its  roof  reaching  the  line  of  the 
springers  of  the  arch,  and  its  floor  being  about  3  ft.  higher  than 
that  of  the  tunnel.     With  the  excavations  of  the  part  No.  2, 
the  drift  was  widened  on  each  side, 
-except  at  the  roof,  and  the  floor  of 
the  tunnel  was  reached.     Above  the 
drift,  the  heading  No.  3  was  exca- 
vated, and  when    the    parts  No.  4 
were  battered  down  the  excavation 
of  the  upper  portion  of  the  tunnel 
section  was  completed,  and  the  ma- 
sonry arch  of  the  lining  built.     The 
parts    No.    5    were    afterward    re- 
moved, and  the  side  walls  built  up 
from  foundations,  and  the  arch  un- 
derpinned.    In   the    middle   of  the 
floor,  the  part  No.  6  was  excavated,  and  the  culvert  built. 

The  excavation  of  the  Mont  Cenis  tunnel  was  carried  on 
by  hand  labor  up  to  the  year  1861,  when  the  first  drilling 
machine  was  employed.  The  drift,  when  excavated  by  hand 
labor,  was  blasted  by  means  of  many  charges  placed  in  holes  no 
more  than  l£  ft.  deep,  and  very  close  together.  When  the  per- 
forating machines  were  first  used  the  same  manner  of  boring 
numerous  shaft-holes  was  followed,  and  the  drift  was  excavated 
by  a  circular  cut.  Near  the  center  13  holes  were  driven,  which 
formed  the  first  round  of  blasting ;  close  to  sides  16  holes  were 
bored  on  each  side  for  the  second  round ;  8  holes  below  and  13 
above  the  circular  cut  formed  the  third  round ;  and  close  to  the 


FIG.  55.— Diagram  Showing  Se- 
quence of  Excavation  in  Mont 
Cenis  Tunnel. 


90  TUNNELING 

floor  5  more  holes  were  bored  for  the  fourth  round,  by  which 
the  floor  of  the  drift  was  reached.  The  total  number  of  the 
holes  bored  at  the  front  of  the  drift  varied  from  70  to  80 ;  their 
depth  was  3£  ft.  Three  holes  in  the  middle  of  those  of  the 
first  round  were  made  deeper  so  as  to  loosen  the  rock  a  little  to 
facilitate  the  blasting  of  the  succeeding  rounds.  Gunpowder 
was  the  only  explosive  used  in  the  excavation  of  the  Mont 
Cenis  tunnel,  both  when  the  work  was  done  by  hand  labor  and 
by  machines. 

The  time  required  for  boring  the  holes  of  the  drift  varied 
between  6  and  8  hours.  From  1£  to  2  hours  were  required  for 
filling  in  the  holes  with  explosives,  and  from  3  to  5  hours  in 
removing  the  blasted  rock,  so  that  in  24  hours  no  more  than 
two  blasts  were  made  at  the  front  of  the  drift.  The  different 
excavations  were  made  by  various  gangs  following  each  other 
at  an  average  distance  of  900  ft. 

Power  Plant.  —  The  mechanical  installation  consisted  of 
the  Sommeilier  air  compressors  built  near  the  portals.  The 
Sommeilier  compressors,  Mr.  W.  L.  Saunders  says,  were  oper- 
ated as  a  ram,  utilizing  a  natural  head  of  water  to  force  air  at 
80  Ibs.  pressure  into  a  receiver.  The  column  of  water  con- 
tained in  the  long  pipe  on  the  side  of  the  hill  was  started  and 
stopped  automatically  by  valves  controlled  by  engines.  The 
weight  and  momentum  of  the  water  forced  a  volume  of  air  with 
such  a  shock  against  the  discharge  valve  that  it  was  opened, 
and  the  air  was  discharged  into  the  tank ;  the  valve  was  then 
closed,  the  water  checked ;  a  portion  of  it  was  allowed  to  dis- 
charge, and  the  space  was  filled  with  air,  which  was  in  turn 
forced  into  the  tank.  Only  73  %  of  the  power  of  the  water  was 
available,  27  %  being  lost  by  the  friction  of  the  water  in  the 
pipes,  valves,  bends,  etc.  Of  the  73  %  of  net  work,  49.4  was 
consumed  in  the  perforators,  and  23.6  in  a  dummy  engine 
for  working  the  valves  of  the  compressors  and  for  special 
ventilation. 

The  compressed  air  was  conveyed  from  each  end  through  a 


TUNNELS   THROUGH   HARD   ROCK  91 

cast-iron  pipe  7|  in.  in  diameter,  up  to  the  front  of  the  excava- 
tion. The  joints  of  the  pipes  were  made  with  turned  faces,, 
grooved  to  receive  a  ring  of  oakum  which  was  tightly  screwed 
and  -compressed  into  the  joint.  To  ascertain  the  amount  of 
leakage  of  the  pipes,  they  and  the  tanks  were  filled  with  air 
compressed  to  6  atmospheres,  and  the  machines  stopped  ;  after 
12  hours  the  pressure  was  reduced  to  5.7  atmospheres,  or  to 
95  %  of  the  original  pressure. 

Sommeilier's  percussion  drilling  machines  were  used  in  the 
excavation  of  this  tunnel.  They  were  provided  with  8  or  10- 
drills  acting  at  the  same  time,  and  mounted  on  carriages  running 
on  tracks.  These  were  withdrawn  to  a  safe  place  during  the 
blasting,  and  advanced  again  after  the  broken  rock  was  removed 
from  the  front  and  the  new  tracks  laid. 

Machine  shops  were  built  at  both  ends  of  the  tunnel  for 
building  and  repairing  the  drilling  machines,  bits,  tools,  etc. 
A  gas  factory  was  built  at  each  end  for  lighting  purpose. 

Strutting.  —  The  roof  of  the  drift  was  strutted  by  means  of 
longitudinal  planks  supported  by  cap-pieces  laid  across  the  line 
of  the  tunnel  and  resting  on  vertical  props  close  to  the  sides 
of  the  excavation.  This  strutting  was  necessitated  in  order  to 
prevent  the  fall  of  the  rock  from  the  upper  part  of  the  section. 
For  the  upper  portion  of  the  profile  no  continuous  strutting 
was  required,  but  at  places  where  the  rock  was  fissured  or 
disintegrated  a  polygonal  strutting  was  employed.  This 
consisted  of  a  sill  laid  across  the  axis  of  the  tunnel  and  just 
above  the  roof  of  the  drift  On  this  sill  two  inclined  props 
were  placed  supporting  a  cap-piece.  Close  to  the  feet  of  these 
two  inclined  props  other  props  were  inserted  abutting  against 
wooden  blocks  close  to  the  faces  of  the  excavation.  These 
blocks  were  of  trapezoidal  shape,  the  smaller  side  being  near 
the  excavation,  while  the  longer  ones  abutted  against  the 
props.  Between  two  consecutive  wooden  blocks  small  beams 
were  inserted  as  close  as  possible  to  the  excavation,  and  in 
such  a  manner  as  to  assume  the  form  of  a  polygon.  Planks- 


92  TUNNELING 

were  stretched  longitudinally  between  the  beams  forming  the 
polygons  of  the  consecutive  timber  structures. 

Masonry.  —  After  the  upper  portion  of  the  tunnel  section 
had  been  excavated,  the  arch  was  built  with  its  feet  resting 
upon  heavy  planks.  For  the  construction  of  the  arch  light 
centers  were  used.  The  arch  was  made  of  brick,  and  rested  on 
the  unexcavated  portions  of  the  bench.  When  these  were 
removed,  pillars  of  rock  from  6  to  8  ft.  long  were  left  at  equal 
intervals  between  them.  In  the  spaces  left  vacant,  balks  of 
timber  were  inserted  in  order  to  support  the  arch.  In  the 
space  between  the  rock  pillars  the  side  walls  were  built  up 
from  foundation  and  the  arch  underpinned ;  then  the  rock  pillars 
were  in  their  turn  battered  down,  new  timbers  were  inserted 
to  support  the  arch,  and  the  side  walls  were  built  and  the  arch 
underpinned.  In  this  way  the  masonry  of  the  lining  was  made 
continuous.  At  every  3,000  ft.  large  niches  were  built,  while 
all  along  the  line  on  both  sides  small  sheltering  niches  were 
built  150  ft.  apart. 

Hauling.  —  In  the  Mont  Cenis  tunnel  all  the  hauling  was 
done  by  horses.  On  the  floor  of  the  drift  small  tracks  were 
placed,  upon  which  ran  the  cars  that  removed  the  broken  rock 
produced  by  blasting  at  the  front.  At  the  end  of  the  drift  the 
small  cars  dumped  the  rock  into  larger  cars  running  on  the 
floor  of  the  part  No.  2  which  was  the  tunnel  floor.  There  a 
single  track  was  laid,  which  was  afterward  switched  into  a 
double  track  where  the  full  section  of  the  tunnel  was  opened. 
The  materials  excavated  from  the  upper  portion  of  the  profile, 
by  means  of  openings  left  in  the  roof  of  the  drift,  were  loaded 
directly  on  to  the  large  cars  running  on  the  tunnel  floor. 

Ventilation.  —  Ventilation  was  at  first  obtained  by  the  air 
discharged  from  the  drills,  which  exhausted  from  250,000  to 
280,000  cu.  ft.  of  fresh  air  every  hour  at  the  front.  When  this 
quantity  was  considered  too  small,  a  blower  2.5  ft.  in  diameter 
was  employed.  It  was  operated  by  a  small  compressed  air 
motor,  and  the  air  was  driven  to  the  front  through  a  10  in.  box 


TUNNELS    THROUGH    HARD    ROCK  9S 

conduit  of  square  section.  When  the  work  was  well  advanced 
this  apparatus  was  deemed  to  be  insufficient,  and  the  exhaust- 
ing bells  described  in  the  Chapter  Ventilation  were  used,  and 
operated  by  a  powerful  turbine,  whose  motive-power  was  a 
stream  of  75  gallons  of  water  per  second  with  a  head  of  60  ft. 


94  TUNNELING 


CHAPTER  X. 

TUNNELS    THROUGH    HARD    ROCK   (Continued), 
THE    SIMPLON    TUNNEL.* 


BEFORE  entering  upon  a  description  of  the  constructive 
details  of  this,  the  longest  railway  tunnel  in  the  world,  it  may 
be  well  to  give  a  general  idea  of  the  undertaking.  Many 
schemes  for  the  connection  of  Italy  and  Switzerland  by  a  rail- 
way near  the  Simplon  Road  Pass  have  been  devised,  including 
one  involving  no  great  length  of  underground  work,  the  line 
mounting  by  steep  gradients  and  sharp  curves.  The  present 
scheme,  put  forward  in  1881  by  the  Jura-Simplon  Ry.  Co.,  con- 
sists broadly  of  piercing  the  Alps  between  Brigue,  the  present 
railway  terminus  in  the  Rhone  Valley,  and  Iselle,  in  the 
gorge  of  the  Diveria,  on  the  Italian  side,  from  which  village 
the  railway  will  descend  to  the  existing  southern  terminus  at 
Domo  d'Ossola,  a  distance  of  about  11  miles. 

In  conjunction  with  this  scheme  a  second  tunnel  is  pro- 
posed, to  pierce  the  Bernese  Alps  under  the  Lotschen  Pass 
from  Mittholz  to  a  point  near  Turtman  in  the  Rhone  Valley ; 
and  thus,  instead  of  the  long  detour  by  Lausanne  and  the  Lake 
of  Geneva,  there  will  be  an  almost  direct  line  from  Berne  to 
Milan  via  Thun,  Brigue,  and  Domo  d'Ossola. 

Starting  from  Brigue,  the  new  line,  running  gently  up 
the  valley  for  1J  miles,  will,  on  account  of  the  proximity  of 
the  Rhone,  which  has  already  been  slightly  diverted,  enter  the 
tunnels  on  a  curve  to  the  right,  of  1,050  ft.  radius.  At  a 
distance  of  153  yards  from  the  entrance,  the  straight  portion 

*  Abstract  from  a  paper  read  before  the  Institution  of  Civil  Engineers  by  Charles  B. 
Tox,  Jan.  26,  1900. 


TUNNELS    THROUGH   HARD   ROCK  95 

of  the  tunnel  commences,  and  extends  for  12  miles.  The  line 
then  curves  to  the  left  with  a  radius  of  1,311  ft  before  emerging 
on  the  left  bank  of  the  Diveria.  Commencing  at  the  northern 
entrance,  a  gradient  of  1  in  500  (the  minimum  for  efficient 
drainage)  rises  for  a  length  of  5^  miles  to  a  level  length  of 
550  yards  in  the  center,  and  then  a  gradient  of  1  in  143  de- 
scends to  the  Italian  side.  On  the  way  to  Domo  d'Ossola  one 
helical  tunnel  will  be  necessary,  as  has  been  carried  out  on  the 
St.  Gothard.  There  will  be  eventually  two  parallel  tunnels, 
having  their  centers  56  ft.  apart,  each  carrying  one  line  of  way; 
but  at  the  present  time  only  one  heading,  that  known  as  No.  1, 
is  being  excavated  to  full  size,  No.  2  being  left,  masonry  lined 
where  necessary,  for  future  developments.  By  means  of  cross 
headings  every  220  yds.  the  problems  of  transport  and  ventila- 
tion are  greatly  facilitated,  as  will  be  seen  later.  As  both 
entrances  are  on  curves,  a  small  "gallery  of  direction"  is 
necessary,  to  allow  corrections  of  alinement  to  be  made  direct 
from  the  two  observatories  on  the  axis  of  the  tunnel. 

The  outside  installations  are  as  nearly  in  duplicate  as  cir- 
cumstances will  allow,  and  consist  of  the  necessary  offices, 
workshops,  engine-sheds,  power-houses,  smithies,  and  the  nu- 
merous buildings  entailed  by  an  important  engineering  scheme. 
Great  care  is  taken  that  the  miners  and  men  working  in  the 
tunnel  shall  not  suffer  from  the  sudden  change  from  the  warm 
headings  to  the  cold  Alpine  air  outside ;  and  for  this  purpose 
a  large  building  is  in  course  of  erection,  where  they  will  be 
able  to  take  off  their  damp  working  clothes,  have  a  hot  and 
cold  douche,  put  on  a  warm  dry  suit,  and  obtain  refreshments 
at  a  moderate  cost  before  returning  to  their  homes.  Instead 
of  each  man  having  a  locker  in  which  to  stow  his  clothes,  a 
perfect  forest  of  cords  hangs  down  from  the  wooden  ceiling, 
25  ft.  above  floor-level,  each  cord  passing  over  its  own  pulleys 
and  down  the  wall  to  a  numbered  belaying-pin.  Each  cord 
supports  three  hooks  and  a  soap-dish,  which,  when  loaded  with 
their  owner's  property,  are  hauled  up  to  the  ceiling  out  of  the 


96  TUNNELING 

way.  There  are  2,000  of  these  cords,  spaced  1  ft.  6  ins.  apartr 
one  to  each  man.  The  engineers  and  foremen  are  more  priv- 
ileged, being  provided  with  dressing-rooms  and  baths,  partitioned 
off  from  the  two  main  halls.  An  extensive  clothes  washing 
and  drying  plant  has  been  laid  down,  and  also  a  large  restau- 
rant and  canteen.  At  Iselle,  a  magazine  holding  2,200  Ibs.  of 
dynamite  is  surrounded  and  divided  into  two  separate  parts  by 
earth-banks,  16  ft.  high.  The  two  wooden  houses,  in  which 
the  explosive  is  stored,  are  warmed  by  hot-water  pipes  to  a 
temperature  between  61°  F.  and  77°  F.,  and  are  watched  by 
a  military  patrol;  but  at  Brigue  a  dynamite  manufactory, 
started  by  an  enterprising  company  at  the  time  of  the  com- 
mencement of  the  works,  supplies  this  commodity  at  frequent 
intervals,  thereby  avoiding  the  necessity  of  storing  in  such 
large  quantities.  This  dynamite  factory  has  been  largely  in- 
creased, and  supplies  dynamite  to  nearly  all  the  mining  and 
tunneling  enterprises  in  Switzerland. 

Geological  Conditions.  —  Before  the  Simplon  tunnel  was  au- 
thorized, expert  evidence  was  taken  as  to  the  feasibility  of 
the  project.  The  forecasts  of  the  three  engineers  chosen, 
in  reference  to  the  rock  to  be  encountered  and  its  probable 
temperature,  have,  as  far  as  the  galleries  have  gone  (an  ag- 
gregate distance  of  nearly  2j  miles),  generally  been  found 
correct.  At  the  north  end,  a  dark  argillaceous  schist  veined 
with  quartz  was  met  with,  and  from  time  to  time  beds  of 
gypsum  and  dolomite  have  been  traversed,  the  dip  of  the 
strata  being  on  the  whole  favorable  to  progress,  though  timber- 
ing is  resorted  to  at  dangerous  places.  Water  was  plentiful 
at  the  commencement ;  in  fact,  one  inrush  has  not  been  stopped, 
and  is  still  flowing  down  the  heading.  The  total  quantity  of 
water  flowing  from  the  tunnel  mouth  is  16  gallons  per  second, 
of  which  2  gallons  per  second  are  accounted  for  by  the  drilling 
machines.  At  Iselle,  however,  a  very  hard  antigorio  gneiss 
obtains,  and  is  likely  to  extend  for  4  miles.  Very  dry  and 
very  compact,  it  requires  no  timbering,  and  presents  no  great 


TUNNELS    THROUGH    HARD   ROCK  97 

difficulty  to  the  powerful  Brandt  rock-drills,  which  work  under 
a  head  of  3,280  ft.  of  water. 

The  temperature  of  the  rock  depends  not  only  on  the  depth 
from  the  surface,  but  largely  upon  the  general  form  of  that  sur- 
face combined  with  the  conductivity  of  the  rock.  Taking 
these  points  into  consideration  with  the  experience  gained  from 
the  construction  of  the  St.  Gothard  tunnel,  95°  F.  was  esti- 
mated as  the  probable  maximum  temperature,  owing  to  the 
height  of  Monte  Leone  (11,660  ft),  which  lies  almost  directly 
over  the  tunnel  axis. 

Survey —  After  having  determined  upon  the  general  position 
of  the  tunnels,  taking  into  consideration  the  necessary  gra- 
dients, the  temperature  of  the  rock,  and  a  large  bed  of  trouble- 
some gypsum  on  the  north  side,  two  fixed  points  on  the 
proposed  center  line  were  taken,  one  at  each  entrance  of  tunnel 
No.  1,  and  the  bearings  of  these  two  points,  with  reference  to 
a  triangulation  survey  made  in  1876,  were  calculated  sufficiently 
accurately  to  determine,  for  the  time  being,  the  direction  of 
the  tunnel.  In  1898,  a  new  triangulation  survey  was  made, 
taking  in  eleven  summits,  Monte  Leone  holding  the  central 
position.  This  survey  was  tied  into  that  of  the  Wasenhorn 
and  Faulhorn,  made  by  the  Swiss  Government,  and  the  accuracy 
was  such  that  the  probable  error  in  the  meeting  of  the  two 
headings  is  only  6  cms.  or  2^-  ins. 

On  the  top  of  each  summit  is  placed  a  signal,  consisting  of 
a  small  pillar  of  masonry  founded  on  rock,  and  capped  with  a 
sharp  pointed  cone  of  zinc,  1  ft.  6  ins.  high.  An  observatory 
was  built  at  each  end  of  the  tunnel  in  such  a  position  that  three 
of  the  summits  could  be  seen,  a  condition  very  difficult  to  fulfill 
on  the  south  side  owing  to  the  depth  of  the  gorge,  the  moun- 
tains on  either  side  being  over  7,000  ft.  high.  Having  taken 
the  angles  to  and  from  each  visible  signal,  and  therefrom  having 
calculated  the  direction  of  the  tunnel,  it  was  necessary  to  fix, 
with  extreme  accuracy,  sighting-points  on  the  axis  of  the  tunnel, 
in  order  to  avoid  sighting  on  to  the  surrounding  peaks  for  each 


98  TUNNELING 

subsequent  correction  of  the  alinement  of  the  galleries.  To 
do  this,  a  theodolite  24  ins.  long  and  2f  ins.  in  diameter, 
with  a  magnifying  power  of  40  times,  was  set  up  in  the  observ- 
atory, and  about  100  readings  were  taken  of  the  angles  between 
the  surrounding  signals  and  the  required  sighting-points.  In 
this  manner  the  error  likely  to  occur  was  diminished  to  less 
than  1'.  Thus  at  the  north  end  two  points  were  found  about 
550  yds.  before  and  behind  the  observatory,  while  on  the  south 
side,  owing  to  the  narrowness  of  the  gorge,  the  points  could 
only  be  placed  at  82  yds.  and  126  yds.  in  front.  One  of  these 
sighting-points  consists  of  a  fine  scratch  ruled  on  a  piece  of  glass 
fixed  in  an  iron  frame,  behind  which  is  placed  an  acetylene 
lamp,  —  corrections  of  alinement  are  always  done  by  night,  — 
the  whole  being  rigidly  fixed  into  a  niche  cut  in  the  rock  and 
protected  from  climatic  and  other  disturbing  agencies  by  an 
iron  plate. 

Method  of  Checking  Alinement.  —  The  direction  of  heading 
No.  1  is  checked  by  experts  from  the  Government  Survey  De- 
partment at  Lausanne  about  three  times  a  year,  and  for  this 
purpose  a  transit  instrument  is  set  up  in  the  observatory.  A 
number  of  three-legged  iron  tables  are  placed  at  intervals  of 
1  mile  or  2  miles  along  the  axis  of  tunnel  No.  1,  and  upon 
each  of  these  is  placed  a  horizontal  plane,  movable  by  means  of 
an  adjusting  screw,  in  a  direction  at  right  angles  to  the  axis, 
along  a  graduated  scale.  On  this  plane  are  small  sockets,  into 
which  the  legs  of  an  acetylene  lamp  and  screen,  or  of  the 
transit  instrument,  can  be  quickly  and  accurately  placed.  The 
screen  has  a  vertical  slit,  3  ins.  in  height,  and  variable  between 
|f  in.  and  ^\  in.  in  breadth,  according  to  the  state  of  the  atmos- 
phere, and  at  a  distance  shows  a  fine  thread  of  light.  The 
instrument,  having  first  been  sighted  on  to  the  illuminated 
scratch  of  the  sighting-point,  is  directed  up  the  tunnel,  where  a 
thread  of  light  is  shown  from  the  first  table.  With  the  aid  of 
a  telephone  this  light  is  adjusted  so  that  its  image  is  exactly 
coincident  with  the  cross  hairs,  and  the  reading  on  the  gradu- 


TUNNELS   THROUGH   HARD    BOCK  99 

ated  scale  is  noted.  This  is  done  four  or  five  times,  the  aver- 
age of  these  readings  being  taken  as  correct,  and  the  plane  is 
clamped  to  that  average.  The  instrument  is  then  taken  to  the 
first  table  and  is  placed  quickly  and  accurately  over  the  jflpint 
just  found  (by  means  of  the  sockets),  and  the  lamp  is  carried 
to  the  observatory.  After  first  sighting  back,  a  second  point  is 
given  on  the  second  table,  and  so  on.  These  points  are  marked 
either  temporarily  in  the  roof  of  the  heading  by  a  short  piece 
of  cord  hanging  down,  or  permanently  by  a  brass  point  held  by 
a  small  steel  cylinder,  8  ins.  long  and  3  ins.  in  diameter,  em- 
bedded in  concrete  in  the  rock  floor,  and  protected  by  a  circular 
casting,  also  sunk  in  cement  concrete,  holding  an  iron  cover 
resembling  that  of  a  small  manhole.  From  time  to  time  the 
alinement  is  checked  from  these  points  by  the  engineers,  and 
after  each  blast  the  general  direction  is  given  by  the  hand  from 
the  temporary  points.  To  check  the  results  of  the  triangula- 
tion  survey,  astronomical  observations  have  been  taken  simul- 
taneously at  each  end.  With  regard  to  the  levels,  those  given 
on  the  excellent  Government  surveys  have  been  taken  as  cor- 
rect, but  they  have  also  been  checked  over  the  pass. 

Details  of  Tunnels.  —  In  cross-section,  tunnel  No.  1  is  13  ft. 
7  ins.  wide  at  formation  level,  increasing  to  16  ft.  5  ins.,  with 
a  total  height  of  18  ft.  above  rail-level,  and  a  cross-sectional 
area  of  about  250  sq.  ft.  This  large  section  will  allow  of 
small  repairs  being  executed  in  the  roof  without  interruption 
of  the  traffic,  and  will  also  allow  of  strengthening  the  walls  by 
additional  masonry  on  the  inside.  The  thickness  of  the  lining, 
never  wholly  absent,  and  the  material  of  which  it  is  composed, 
depend  upon  the  pressure  to  be  resisted,  and  only  in  the  worst 
case  is  an  invert  resorted  to.  The  side  drain,  to  which  the  rock 
floor  is  made  to  slope,  will  be  composed  of  half-pipes  of  7  to  1 
cement  concrete.  The  roof  is  constructed  of  radial  stones. 

Tunnel  No.  2,  being  left  as  a  heading,  is  driven  on  that  side 
nearest  to  No.  1,  to  minimize  the  length  of  the  cross-headings, 
and  measures  10  ft  2  ins.  wide  by  6  ft.  7  ins.  high.  Masonry 


100  TUNNELING 

is  used  only  where  necessary,  and  in  that  case  is  so  built  as  to 
form  part  of  the  lining  of  the  tunnel  when  eventually  com- 
pleted. Concrete  is  put  in  to  form  a  foundation  for  the  side 
wall,  and  a  water  channel.  The  cross-headings,  connecting  the 
two  parallel  headings,  occur  every  220  yds.,  and  are  placed  at 
an  angle  of  56°  to  the  axis  of  the  tunnel,  to  avoid  sharp  curves 
in  the  contractors'  railway  lines.  They  will  eventually  be  used 
as  much  as  possible  for  refuges,  chambers  for  storing  the  tools 
and  equipment  of  the  platelayers,  and  signal-cabins.  The  ref- 
uges, 6  ft.  7  ins.wide  by  6  ft.  7  ins.  high  and  3  ft.  3  ins.  deep, 
occur  every  110  yards,  every  tenth  being  enlarged  to  9  ft.  10 
ins.  wide  by  9  ft.  10  ins.  deep  and  10  ft.  2  ins.  high,  still  larger 
chambers  being  constructed  at  greater  intervals. 

Method  of  Excavation.  —  The  work  at  each  end  of  the  tunnel 
is  carried  on  quite  independently,  consequently,  though  similar 
in  principle,  the  methods  vary  in  detail,  apart  from  the  fact  that 
different  geological  strata  require  different  treatment.  Broadly 
speaking,'  the  two  parallel  headings,  each  59  sq.  ft.  in  section, 
are  first  driven  by  means  of  drilling-machines  and  the  use  of 
dynamite,  this  work  being  carried  on  day  and  night,  seven  days. 
in  the  week;  No.  1  heading  is  then  enlarged -to  full  size  by 
hand-drilling  and  dynamite.  On  the  Italian  side,  where  the 
rock  is  hard  and  compact,  breakups  are  made  at  intervals  of 
50  yds.,  and  a  top  gallery  is  driven  in  both  directions,  but,  for 
ventilation  reasons,  is  never  allowed  to  get  more  than  4  yds. 
ahead  of  the  breakup,  which  is  gradually  lengthened  and 
widened  to  the  required  section.  No  timbering  is  required, 
except  to  facilitate  the  excavation  and  the  construction  of  the 
side  walls.  Steel  centers  are  employed  for  the  arch ;  they  entail 
fewer  supports,  give  more  room,  and  are  capable  of  being  used 
over  again  more  frequently,  without  damage.  They  consist 
of  two  I-beams  bent  to  a  template  and  riveted  together  at  the 
crown,  resting  at  either  side  on  scaffolding  at  intervals  of  6  ft. ; 
longitudinals,  12  ft.  by  4  ins.  by  4  ins.,  support  the  roof.  Hand 
rock-drilling  is  carried  out  in  the  ordinary  way,  one  man  holding 


TUNNELS  THROUGH  HARD  ROCK 


101 


the  tool  and  a  second 
striking ;  measure- 
ments of  excavation 
are  taken  every  2  or 
3  yds.,  a  plumb-line  is 
suspended  from  the 
center  of  the  roof,  and 
at  every  half-meter 
(20  ins.)  of  height 
horizontal  measure- 
ments are  taken  to 
each  side. 

At  the  Brigue  end 
a  softer  rock  is  en- 
countered, necessitate 
ing  at  times  heavy 
timbering  in  the  head- 
ing, and  especially  in 
the  final  excavation 
to  full  size,  Fig.  56. 
The  bottom  heading, 
6  ft.  6  in.  high,  is 
driven  in  the  center, 
and  the  heading  is 
then  widened  to  the 
full  extent  and  tim- 
bered ;  the  concrete 
forming  the  water 
channel  and  the  foun- 
dation for  one  side 
wall  is  put  in ;  the 

side  walls  are  built  to  a  height  of  6  ft.  6  ins.,  and  the  tunnel 
is  fully  excavated  to  a  further  height  of  6  ft.  6  ins.  from  the 
first  staging.  The  side  walls  are  then  continued  up  for  the 
second  6  ft.  6  ins.,  and  from  the  second  floor  a  third  height  of 


FlO.   56.  — Sketches  Showing  Sequence  of  Work  in 
Excavating  and  Lining  the  Simplon  Tunnel. 


102  TUNNELING 

6  ft.  6  ins.  is  excavated  and  timbered.  Finally  the  crown  is 
cleared  out,  heavy  wooden  centers  are  put  in,  the  arch  is  turned, 
and  all  timbers  are  withdrawn  except  the  top  poling-boards, 
supporting  the  loose  rock. 

The  masonry  for  the  side  walls  is  obtained  either  from  the 
tunnel  itself  or  from  a  neighboring  quarry,  and  varies  in  char- 
acter according  to  the  pressure ;  but  the  face  of  the  arch  is  al- 
ways of  cut  or  artificial  stones,  the  latter  being  of  7  to  1  cement 
concrete.  Where  the  alinement  heading,  or  the  "gallery  of 
direction,"  joins  the  curving  portion  of  tunnel  No.  1,  the  section 
is  very  much  greater,  and  necessitates  special  timbering. 

Transport  (Italian  Side).  —  A  small  line  of  railway,  2  ft.  7j 
ins.  gauge,  with  40-lb.  rails,  enters  all  three  portals ;  but  since 
the  construction  of  a  wooden  bridge  over  the  Diveria,  the  route 
through  the  "gallery  of  direction,"  across  heading  No.  2,  to 
tunnel  No.  1,  is  used  exclusively;  this  railway  leads  to  the  face 
in  both  headings,  and,  where  convenient,  from  one  heading  to 
the  other  by  the  cross-galleries.  Different  types  of  wagons  are 
in  use ;  but  in  general  they  are  four-wheeled,  non-tipping  box 
wagons,  supplied  with  brakes  and  holding  2  cu.  yds.  of  debris. 
A  special  type  of  locomotive  is  used,  designed  to  pass  round 
curves  of  50  ft.  radius,  and  supplied  with  a  specially  large  boiler 
to  avoid  firing  in  the  tunnel. 

Method  of  Working.  —  The  drilling-machines  employed  are  of 
the  Brandt  type,  Fig.  57,  and  are  mounted  in  the  following 
manner:  A  small  four-wheeled  carriage  supports  at  its  center 
a  beam,  the  shorter  arm  of  which  carries  the  boring  mechanism 
and  the  longer  a  counterpoise ;  near  its  center  is  the  distributor. 
In  the  short  arm  is  a  clamp  holding  the  rack-bar  or  butting 
column,  which  is  a  wrought-iron  cylinder  with  a  plunger  con- 
stituting a  ram,  and  is  jammed  by  hydraulic  pressure  between 
the  walls  of  the  heading,  thus  forming  a  rigid  support,  for  the 
boring-machine,  and  an  efficient  abutment  against  the  i  eaction 
of  the  drill.  This  rack-bar  can  be  rotated  on  its  cl  inp  in  a 
plane  parallel  to  the  axis  of  the  beam.  Three  or  four  separate 


TUNNELS  THROUGH  HARD  ROCK          103 

boring-machines  can  be  mounted  on  the  rack-bar,  and  can  be 
adjusted  in  any  reasonable  position. 

The  boring-machine  performs  the  double  function  of  con- 
tinually pressing  the  drill  into  the  rock  by  means  of  a  hallow 
ram  (1),  and  of  imparting  to  the  drill  and  ram  a  uniform  rotary 
motion.  This  rotary  motion  is  given  by  a  twin  cylinder  single- 
acting  hydraulic  motor  (^),  the  two  pistons,  of  2|  ins.  stroke, 
acting  reciprocally  as  valves.  The  cranks  are  fixed  at  an  angle 
of  90°  to  each  other  on  the  shaft,  which  carries  a  worm,  gearing 
with  a  worm-wheel  ($)  mounted  upon  the  shell  (jR)  of  the 


FIG.  57.  —  General  Details  of  the  Brandt  Rotary  Drills  Employed  at  the  Simplon  Tunnel. 

hollow  ram  (1),  and  this  shell  in  turn  engages  the  ram  by  a 
long  feather,  leaving  it  free  to  slide  axially  to  or  from  the  face 
of  the  rock.  The  average  speed  of  the  motor  is  150  revolutions 
to  200  revolutions  per  minute,  the  maximum  speed  being  300 
revolutions  per  minute.  The  loss  of  power  between  the  worm 
and  worm-wheel  is  only  15  °f0  at  the  most;  the  worm  being  of 
hardened  steel  and  the  wheel  of  gun-metal,  the  two  surfaces  in 
contact  acquire  a  high  degree  of  polish,  resulting  in  little  wear- 
ing or  heating.  Taking  into  consideration  all  other  sources  of 
loss,  70  %  of  the  total  power  is  utilized.  The  pressure  on  the 


104  TUNNELING 

drill  is  exerted  by  a  cylinder  and  hollow  ram  (J),  which  revolves 
about  the  differential  piston  ($),  which  is  fixed  to  the  envelope 
holding  the  shell  (.#).  This  envelope  is  rigidly  connected  to 
the  bed-plate  of  the  motor,  and,  by  means  of  the  vertical  hinge 
and  pin  (T),  is  held  by  the  clamp  (F')  embracing  the  rack-bar. 
When  water  is  admitted  to  the  space  in  front  of  the  differential 
piston  the  ram  carrying  the  drilling-tool  is  thrust  forward,  and 
when  admitted  to  the  annular  space  behind  the  piston,  the  ram 
recedes,  withdrawing  the  tool  from  the  blast-hole.  The  drill 
proper  is  a  hollow  tube  of  tough  steel  2|  ins.  in  external  diame- 
ter, armed  with  three  or  four  sharp  and  hardened  teeth,  and 
makes  from  five  to  ten  revolutions  per  minute,  according  to  the 
nature  of  the  rock.  When  the  ram  has  reached  the  end  of  its 
stroke  of  2  ft.  2^  ins.,  the  tool  is  quickly  withdrawn  from  the 
hole  and  unscrewed  from  the  ram;  an  extension  rod  is  then 
screwed  into  the  tool  and  into  the  ram,  and  the  boring  is  con- 
tinued, additional  lengths  being  added  as  the  tool  grinds  for- 
ward; each  change  of  tool  or  rod  takes  about  15  sees,  to  25 
sees,  to  perform.  The  extension  rods  are  forged  steel  tubes, 
fitted  with  four-threaded  screws,  and  having  the  same  external 
diameter  as  the  drill.  They  are  made  in  standard  lengths  of 
2  ft.  8  ins.,  1  ft.  10  ins.,  and  11|  ins.  The  total  weight  of  the 
drilling-machine  is  264  Ibs.,  and  that  of  the  rack-bar  when  full 
of  water  is  308  Ibs.  The  exhaust  water  from  the  two  motor 
cylinders  escapes  through  a  tube  in  the  center  of  the  ram  and 
along  the  bore  of  the  extension  rods  and  drill,  thereby  scouring 
away  the  debris  and  keeping  the  drill  cool ;  any  superfluous 
water  finds  an  exit  through  a  hose  below  the  motors  and  thence 
away  down  the  heading.  The  distributor,  already  mentioned, 
supplies  each  boring-machine  and  the  rack-bar  with  hydraulic 
pressure  from  the  mains,  with  which  connection  is  effected  by 
means  of  flexible  or  articulated  pipe  connections,  allowing  free- 
dom in  all  directions.  The  area  of  the  piston  for  advancing 
the  tool  is  15^  sq.  ins.,  which  under  a  pressure  of  1470  Ibs.  per 
sq.  in.  gives  a  pressure  of  over  10  tons  on  the  tool,  while  for 


TUNNELS   THROUGH   HARD   ROCK  105 

i 

withdrawing  the  tool  2£  tons  is  available.  In  the  rock  found  at 
Iselle,  namely,  antigorio  gneiss,  a  hole  2£  ins.  in  diameter  and 
3  ft  8  ins.  in  length  is  drilled,  normally,  in  12  mins.  to  25  mins. ; 
a  daily  rate  of  advance  of  18  ft.  to  19  ft.  6  ins.  is  made*  in  a 
heading  having  a  minimum  cross-section  of  59  sq.  ft. ;  the  time 
taken  to  drill  ten  to  twelve  holes,  4  ft.  7  ins.  deep,  is  2J  hrs. 

When  the  debris  resulting  from  one  operation  has  been 
sufficiently  cleared  away,  a  steel  flooring,  which  is  provided 
near  the  face  to  enable  shoveling  to  be  more  easily  done,  and 
to  give  an  even  floor  for  the  wheels  of  the  drilling-carriage,  is 
laid  bare  at  the  head  of  the  line  of  rails,  and  the  drilling- 
machines  are  brought  up  on  their  carriage  by  eight  or  ten 
men.  When  advanced  sufficiently  close  to  the  face,  the  rack- 
bar  is  slewed  round  across  the  gallery  and  is  wedged  up  against 
the  rock  sides ;  connection  is  made  between  the  distributor  and 
the  hydraulic  main,  by  means  of  the  flexible  pipe,  and  pressure 
is  supplied  by  a  small  copper  tube  to  the  rack-bar  ram,  thereby 
rigidly  holding  the  machine.  Next,  connections  are  made 
between  the  three  drilling-machines  and  the  distributor,  and  in 
20  mins.  from  the  time  the  machine  was  brought  up  all  three 
drills  are  hard  at  work,  water  pouring  from  the  holes. 

The  noise  of  the  motors  and  grinding- tools  is  sufficient  to 
drown  all  but  shouts ;  and  where  the  extension  rods  do  not  fit 
tightly,  small  jets  of  water  play  in  all  directions,  necessitating 
the  wearing  of  tarpaulins  by  the  men  directing  the  tools. 
Lighting  is  done  wholly  by  small  oil-lamps,  provided  with  a 
hook  to  facilitate  fixing  in  any  crack  in  the  rock ;  electricity 
will  probably  be  used  to  light  that  portion  of  the  tunnel  which 
is  completed. 

Two  men  are  allotted  to  each  drill,  one  to  drive  the  motor, 
the  other  to  direct  and  replenish  the  tool,  one  foreman  and  two 
men  in  reserve  completing  the  gang.  A  small  hammer  is  freely 
used  to  loosen  the  screw  joints  of  the  extension  rods  and  drill. 
A  hole  is  usually  commenced  by  a  two-edged  flat-pointed  tool, 
until  a  sufficient  depth  is  reached  to  prevent  the  circular  tool 


106  TUNNELING 

from  wandering  over  the  face  of  the  rock,  but  in  many  instances 
the  hole  is  commenced  with  a  circular  tool.  The  exhaust 
water  during  this  period  flows  away  by  the  hose  underneath 
the  motor.  In  the  antigorio  gneiss,  ten  to  twelve  holes  are 
drilled  for  each  attack,  three  to  four  in  the  center  to  a  depth  of 
3  ft.  3  ins.,  the  remainder,  disposed  round  the  outside  of  the 
face,  having  a  depth  of  4  ft.  7  in.  The  average  time  taken  to 
complete  the  holes  is  If  hr.  to  2-j-  hrs.  Instead  of  pulverizing 
the  rock,  as  do  the  diamond  drills,  it  is  found  that  the  rock  is 
crushed,  and  that  headway  is  gained  somewhat  in  the  manner 
of  a  circular  saw  through  wood.  The  core  of  rock  inside  the 
tool  breaks  up  into  small  pieces,  and  can  be  taken  out  if 
necessary  when  the  drill  requires  lengthening. 

The  lowest  holes,  inclined  down-wards,  are  full  of  water ; 
consequently  two  detonators  and  two  fuses  are  inserted,  but 
apart  from  this,  water  has  little  effect  on  the  charge.  The 
fuses  of  the  central  holes  are  brought  together  and  cut  off 
shorter  than  those  of  the  outer  holes,  in  order  that  they  may 
explode  first  to  increase  the  effect  of  the  outer  charges.  All 
portable  objects,  such  as  drills,  pipe  connections,  tools,  etc.,  have 
meanwhile  been  carried  back ;  the  steel  flooring  is  covered  over 
with  a  layer  of  debris  to  prevent  injury  from  falling  rock,  and 
to  the  end  of  the  hydraulic  main  is  screwed  a  brass  plug 
pierced  by  five  holes ;  and  immediately  the  explosions  occur  a 
valve  is  opened  in  the  tunnel,  and  five  jets  of  water  play  upon 
the  rock,  laying  the  dust  and  clearing  the  air.  The  necessity 
for  this  was  shown  on  one  occasion  when  this  nozzle  was 
broken  by  the  explosion  and  the  water  had  to  be  turned  off 
immediately  to  avoid  useless  waste ;  on  reaching  the  face,  the 
atmosphere  was  found  to  be  so  highly  charged  with  dust  and 
smoke  that  it  was  impossible  to  distinguish  the  stones  at  the 
feet,  although  a  lamp  had  been  placed  on  the  ground;  and 
despite  the  fact  that  the  air  tube  was  in  full  blast,  the  men  ex- 
perienced great  difficulty  in  breathing.  A  truck  is  now  brought 
up,  and  four  men  clear  a  passage  in  front,  through  the  heap  of 


TUNNELS    THROUGH    HARD    BOCK  107 

debris,  two  with  picks  and  two  with  shovels,  while  on  either 
side  and  behind  are  as  many  men  as  space  will  permit.  The 
stone  is  thrown  either  to  the  sides  of  the  heading  or  into  the 
wagon,  shoveling  being  greatly  aided  by  the  steel  flooring, 
which,  before  the  explosion,  had  been  laid  over  the  rails  for 
nearly  10  yds.  down  the  tunnel  to  receive  the  falling  rock. 
These  steel  plates  are  taken  up  when  cleared,  and  the  wagon 
is  pushed  forward  until  the  drilling-machine  can  be  brought  up 
again,  leaving  the  remaining  debris  at  the  sides  to  be  handled 
at  leisure  during  the  next  attack.  The  roof  and  side  walls  are, 
of  course,  carefully  examined  with  the  pick,  to  discover  and 
detach  any  loose  or  hanging  rock.  The  times  taken  for  each 
portion  of  the  attack  in  this  particular  antigorio  gneiss  are  as 
follows :  Bringing  up  and  adjustment  of  drills,  20  mins. ;  drill- 
ing, between  If  hr.  and  2^-  hrs. ;  charging  and  firing,  15  mins. ; 
clearing  away  debris,  2  hrs. ;  or  for  one  whole  attack,  between 
4£  hrs.  and  54^  hrs.,  resulting  in  an  advance  of  3  ft  9  in.,  or  a 
daily  advance  of  nearly  18  ft. 

From  this  it  appears  that  the  time  spent  in  clearing  away 
the  debris  equals  that  taken  up  in  drilling,  and  it  is  in  this  clear- 
ing that  a  saving  of  time  is  likely  to  be  effected  rather  than  in 
the  process  of  drilling.  Many  schemes  have  been  tried,  such  as- 
a  mechanical  plow  for  making  a  passage ;  at  Brigue,  "  marin- 
age,"  or  clearing  by  means  of  powerful  high-pressure  water-jets, 
directed  down  the  tunnel,  was  tried,  but  the  idea  is  not  yet 
sufficiently  developed. 

Another  series  of  experiments  has  been  tried  at  Brigue 
with  regard  to  the  utilization  of  liquid  air  as  an  explosive 
agent  instead  of  dynamite  ;  and  for  this  purpose  a  plant  has  been 
laid  down,  consisting  of  one  ammonia-compressor,  two  air-com- 
pressors, and  two  refrigerators,  furnishing  TV  gallon  of  liquid 
air  per  hour  at  an  expenditure  of  17  H.P.  The  system  used  is 
that  of  Professor  Linde,  who  himself  directs  the  experiments. 
The  great  difficulty  experienced  is  that  of  shortening  the  interval 
of  time  that  must  elapse  between  the  manufacture  of  th& 


108  TUNNELING 

cartridge  and  its  explosion.  The  liquid  oxygen,  with  which 
the  cartridge,  containing  kieselguhr  (silicious  earth)  and 
paraffin,  is  saturated,  evaporates  very  readily,  losing  power 
every  moment ;  hence  the  effect  of  each  cartridge  cannot  be 
guaranteed,  and  though  it  is  an  exceedingly  powerful  explosive 
when  used  immediately  after  manufacture,  no  practical  result 
has  yet  been  obtained. 

Power  Station.  —  Water  is  abundant  at  either  end,  and  there- 
fore hydraulic  power  is  the  motive  force  employed.  On  the 
Italian  side,  a  dam  5  ft.  high  has  been  thrown  across  the  Diveria 
at  a  point  near  the  Swiss  frontier,  about  3  miles  above  the  site 
of  the  installations.  A  portion  of  the  water  thus  held  back 
enters,  through  regulating  doors  and  gratings,  a  masonry 
channel  leading  to  two  parallel  settling  tanks,  each  111  ft.  by 
16  ft.,  whence,  after  dropping  all  its  sand  and  solid  matter,  the 
now  pure  water  passes  into  the  water-house,  and,  after  flowing 
over  a  dam,  through  a  grating  and  past  the  admission  doors, 
enters  a  metallic  conduit  of  3-ft.  pipes.  Each  of  the  settling 
tanks  and  the  approach  canal  are  provided  with  doors  at  the 
lower  end  leading  direct  to  the  river,  through  which  all  the 
sand  and  solid  matter  deposited  can  be  scoured  naturally  by 
allowing  the  river-water  to  rush  freely  through.  For  this  pur- 
pose the  floor  of  the  basins  is  on  an  average  gradient  of  1  in  30. 
For  a  similar  reason  the  river-bed  just  outside  the  entrance  to 
the  approach  canal  is  lined  with  wooden  planks,  from  which 
the  stones  collecting  behind  the  dam  can  be  scoured  by  allow- 
ing an  iron  flap,  hinged  at  the  bottom,  to  change  its  position 
from  the  vertical  to  the  horizontal  in  a  gap  left  purposely  in  the 
dam,  so  causing  a  rushing  torrent  to  sweep  it  clean. 

The  chief  levels  are : 

Level  of  water  at  dam 794.00  meters  above  sea  level. 

"     in  water-house 703.70      "          "       "       " 

"     at  turbines 618.50      "          "       "       " 

giving  a  total  fall  of  175.20  ms.  or  570  ft.,  and  a  pressure  of 
17.52  atmospheres. 


TUNNELS    THROUGH    HARD    ROCK 


The  quantity  of  water  capable  of  being  taken  from  the 
Diveria  in  winter,  when  the  rivers  which  are  dependent  upon 
the  mountain  snows  for  their  supply  are  at  their  lowest,  ia 
calculated  to  be  352  gallons  per  second.  Thus,  taking"  the 
fall  to  be  diminished  by  friction,  etc.,  to  440  ft.,  and  the  use- 
ful effect  at  70  %,  there  is  obtained  2,000  H.P.  on  the  turbine 
shaft. 

The  metallic  conduit  varies  in  material  according  to  the 
pressure ;  thus  cast-iron  pipes  3  ft.  in  diameter  and  j|  in. 
thick  are  used  up  to  a  pressure  of  '2  atmospheres,  from  which 
point  they  are  of  wrought-iron.  The  cast-iron  portion  has  of 
late  caused  a  good  deal  of  trouble,  owing  to  settlement  of  the 
piers  causing  occasional  bursts,  consequently  a  masonry  pier 
has  been  placed  under  each  joint  of  this  portion.  The  follow- 
ing table  gives  the  thicknesses  and  diameters,  varying  with  the 
pressure : 


WATER 
PRESSURE. 

THICKNESS. 

DIAMETER. 

WEIGHT 
PER  YARD. 

Head  in  Feet. 

Milli- 
meters. 

Inch. 

Feet. 

Inches. 

Lbs.     . 

246                  6 

1 

3 

0 

326 

311 

7 

3 

0 

383 

300 

8 

3 

0 

431 

393 

9 

3 

0 

483 

426 

10 

3 

0 

556 

476 

12 

3 

0 

651 

590 

16 

1 

3 

3i 

977 

This  pipe  is  supported  every  30  ft.  on  small  masonry  piers, 
on  the  top  of  which  is  placed  a  block  of  wood  hollowed  out  to 
receive  the  pipe,  thus  allowing  any  movement  due  to  the  con- 
traction and  expansion  of  the  conduit.  However,  to  prevent 
this  movement  becoming  excessive,  the  pipe  is  passed  at 
intervals  of  300  yds.  to  500  yds.  through  a  cubical  block  of 
masonry  of  13  ft.  side,  strengthened  by  longitudinal  tie-bars. 
Five  bands  of  angle-bar  riveted  round  the  pipe,  with  their 


110  TUNNELING 

flanges  embedded  in  the  masonry,  constitute  a  rigid  fixed  point. 
Straw  mats  are  thrown  over  the  pipe  where  it  is  exposed  to  the 
sun.  The  temperature  of  the  conduit  is  not,  however,  found  to 
vary  greatly,  since  the  pipe  is  kept  full  of  water.  To  supply 
the  rock-drills  with  water  at  a  maximum  pressure  of  100 
atmospheres,  or  1,470  Ibs.  per  sq.  in.,  a  plant  of  four  pairs  of 
high-pressure  pumps  has  been  laid  down,  and  a  still  larger 
addition  is  in  course  of  erection.  At  present,  two  Pelton 
turbines  of  250  H.P.  each,  running  at  170  revolutions  per 
minute,  drive  the  pumps,  by  means  of  toothed  gearing,  at  63 
revolutions  per  minute.  These  pumps  are  of  very  simple  but 
strong  construction,  single  suction  and  double  delivery,  entail- 
ing one  suction  and  one  delivery-valve,  both  heavy  and  both  of 
small  lift.  The  larger  portion  of  the  plunger  has  exactly 
double  the  cross-sectional  area  of  the  smaller  portion,  so  that  in 
the  forward  stroke  half  of  the  water  taken  in  at  the  last 
admission  is  pumped  into  the  high-pressure  mains,  and  at  the 
same  time  a  fresh  supply  of  water  is  sucked  in.  During  the 
backward  stroke  half  of  this  new  supply  is  pumped  into 
the  mains,  and  the  remainder  enters  the  second  chamber,  to 
be  pumped  during  the  next  forward  stroke.  Thus  the  work 
done  in  the  two  strokes  is  practically  the  same.  The  pumps 
are  in  pairs,  and  are  set  at  an  angle  of  90°,  to  insure  uniform 
pressure  and  uniform  delivery  in  the  mains.  Their  size  varies ; 
but  at  Iselle  there  are  three  pairs,  with  a  stroke  of  2  ft.  2^  ins., 
and  the  plungers  of  2  |^  in.  and  If  ins.  (approximately)  in 
diameter,  supplying  1.32  gallons  per  second. 

To  avoid  injury  to  the  valves,  the  water  to  be  pumped  is 
taken  from  a  stream  up  the  mountain  side,  and  is  passed 
through  filter  screens.  The  high-pressure  water,  after  passing 
an  accumulator,  enters  the  tunnel  in  solid  drawn  wrought-iron 
tubes,  3^  ins.  in  internal  diameter,  T\  in.  thick,  and  in  lengths 
of  26  ft.  The  diameter  of  these  mains  varies  with  their  length, 
so  as  to  avoid  loss  of  pressure.  With  the  1,250  yds.  of  tunnel 
now  driven  10  atmospheres  are  lost. 


TUNNELS    THROUGH    HARD    ROCK  111 

At  Brigue  the  installations  are,  as  far  as  possible,  identical. 
The  Rhone  water,  however,  before  reaching  the  water-house,  is 
carried  from  the  filter  basins,  a  distance  of  2  miles,  in  an 
armored  canal  built  upon  the  Hennebique  system,*  the  walls 
and  supporting  beams,  of  cement  concrete,  being  strengthened 
by  internal  tie-bars  of  steel.  The  concrete  struts,  resembling 
balks  of  timber  at  a  distance,  are  occasionally  35  ft.  high  and 
1  ft.  7£  ins.  square.  The  metallic  conduit  is  5  ft.  in  diameter, 
with  a  minimum  flow  of  176  cu.  ft.  per  second  and  a  total  fall 
of  185  ft.  In  case  water-power  should  be  unavailable,  three 
semi-portable  steam  engines,  two  of  80  H.P.  and  one  of  60  H.P., 
are  always  kept  in  readiness  at  each  end  of  the  tunnel,  and  are 
geared  by  belts  to  the  turbine  shaft! 

Ventilation.  —  In  tunneling,  one  of  the  most  important  prob- 
lems to  be  solved  is  that  of  ventilation,  and  it  is  for  this  reason 
that  the  Simplon  tunnel  consists  of  two  parallel  headings  with 
cross  cuts  at  intervals  of  220  yds.  At  Brigue,  a  shaft  164  ft. 
deep  was  sunk  through  the  overlying  rock  until  the  "  gallery  of 
direction"  was  encountered.  Up  this  chimney  the  foul  air  is 
drawn  by  wood  fires,  the  fresh  air  —  a  volume  of  19,000,000 
cu.  ft.  per  day,  or  13,200  cu.  ft.  per  minute  —  entering  by 
heading  No.  2,  penetrating  up  to  the  last  cross  gallery,  and 
returning  by  tunnel  No.  1.  The  entrances  of  No.  1  and  the 
"  gallery  of  direction,"  besides  those  of  all  the  intermediate 
cross  galleries,  are  closed  by  doors.  By  this  arrangement,  how- 
ever, fresh  air  does  not  reach  the  working  faces  ;  therefore  a 
pipe,  8  ins.  in  diameter,  is  led  from  the  fresh  air  in  No.  2  to 
within  15  yds.  of  the  face  of  each  heading,  and  up  this  pipe  a 
draft  of  air  is  induced  by  means  of  a  jet  of  water,  the  volume 
to  each  face  being  800  cu.  ft.  per  minute.  One  single  jet  of 
water  from  the  high-pressure  mains,  with  a  diameter  of  TV  in., 
is  capable  of  supplying  over  1,000  cu.  ft.  of  air  per  minute  at 
the  end  of  160  yds.  of  pipe,  and  during  the  attack  the  men  at 
the  drills  are  in  a  constant  breeze  with  the  thermometer  stand- 

*  Network  of  steel  rods  embedded  in  concrete. 


112  TUNNELING 

ing  at  70°  F.  At  Iselle,  air  is  blown  into  the  entrance  of 
headirg  No.  2  at  the  rate  of  14,100  cu.  ft.  per  minute  by  two 
fans  driven  from  the  turbine  shaft.  This  air  travels  from  the 
fans  along  a  pipe,  18  ins.  in  diameter,  till  a  point  15  yds.  up 
the  tunnel  is  reached,  where  beyond  a  door  the  pipe  narrows  to 
form  a  nozzle  10  ins.  in  diameter.  This  door  is  kept  open  to 
allow  the  outside  air  to  be  induced  up  the  tunnel,  as  the  head- 
ings are  at  present  only  2,500  yds.  long,  giving  a  resistance  of 
not  quite  sufficient  power  to  cause  the  air  to  return.  The  fresh 
air  then  travels  up  No.  2,  crossing  over  the  top  of  the  "  gallery 
of  direction,"  from  which  it  is  shut  off  by  doors,  to  the  last 
cross  gallery,  returning  by  No.  1,  and  finally  leaving  either  by 
the  "  gallery  of  direction  "  or  by  No.  1.  A  system  of  cooling 
the  air  and  driving  it  on  by  means  of  a  large  number  of  water- 
jets  will  be  installed  in  No.  2  where  that  heading  crosses  over 
the  "  gallery  of  direction,"  but  at  present  there  is  no  need  for 
it. 

The  average  temperature  at  the  face  is  73°  F.  during  the 
drilling  operation,  76°  F.  after  firing  the  charges,  and  a  max- 
imum of  80°  F.,  lately  attaining  to  86°  F.  on  the  south  side, 
with  80°  F.  and  85°  F.  before  and  after  firing.  The  tempera- 
ture of  the  rock  is  taken  at  every  110  yds.  in  holes  5  ft.  deep, 
and  shows  a  gradual  increase  according  to  the  depth  of  over- 
laying rock,  to  the  conductivity  of  the  rock,  and  to  the  form  of 
the  mountain  surface.  The  maximum  hitherto  reached  on  the 
north  side  is  68°  F.,  while  on  the  south  side,  although  a  smaller 
distance  has  been  traversed,  it  attains  to  79°  F.,  due  to  the 
more  rapid  increase  in  depth.  Moreover,  the  temperature  of 
the  rock  is  observed  at  the  permanent  stations,  550  yds.  from 
the  entrances,  in  its  relation  to  that  of  the  tunnel  and  outside 
air,  and  though  on  the  north  side  that  of  the  rock  varies  almost 
as  quickly  as  that  of  the  tunnel  air,  on  the  south  it  is  influenced 
very  much  less. 

A  few  statistics  may  be  of  interest  with  regard  to  the  prog- 
ress of  the  last  three  months  (taken  from  the  trimes  trial  report 


TUNNELS    THROUGH    HARD    ROCK  113 

of  January,  1900).  At  Brigue,  where  there  are  three  drilling- 
machines  in  No.  1  and  two  in  the  parallel  heading,  the  total 
length  excavated  was  995  yds.  or  6,409  cu.  yds.  in  89  working 
days,  the  average  cross-sectional  area  being  57  sq.  ft.  This  re- 
quired 507  attacks  and  3,06(5  holes,  which  had  a  total  depth  of 
26,600  ft.,  and  14,700  re-sharpenings  of  the  drilling-tool,  with 
44,000  Ibs.  of  dynamite. 

The  average  time  occupied  in  drilling  was  '2  hrs.  45  mins., 
while  charging,  firing,  and  clearing  away  the  debris  took  6  hrs., 
35  mins.  At  Brigue  648  men  and  29  horses  were  employed  at 
one  time  in  the  tunnel.  At  Iselle  the  numbers  were  496  men 
and  16  horses,  working  in  shifts  of  8  hrs.  Outside  the  tunnel, 
in  the  shops,  forges,  etc.,  the  men  work  8  hrs.  to  11  hrs.  per 
day,  the  total  being  541  men  at  Biigue  and  346  men  at  Iselle. 
On  the  Italian  side,  where  the  rock  is  very  much  harder,  there 
were  three  drilling-machines  in  each  heading ;  the  total  length 
excavated,  with  a  cross-sectional  area  of  62  sq.  ft.,  was  960  yds. 
or  6,700  cu.  yds.  in  91  working  days.  This  required  61,293 
re-sharpened  tools,  758  attacks,  7,940  holes  with  a  total  depth 
of  33,000  ft,,  and  56,000  Ibs.  of  dynamite.  The  average  time 
spent  in  drilling  was  2  hrs.  55  mins.,  and  in  charging  and  clear 
ing  2  hrs.  36  mins.  Thus,  in  the  hard  gneiss,  to  excavate  1  cu. 
yd.  of  rock  required  8£  Ibs.  of  dynamite,  and  each  tool  pierced 
6£  ins.  of  rock  before  it  required  re-sharpening. 

Up  to  January  1,  1900,  the  total  length  of  heading  on  the 
north  side  was  2,515  yds.,  and  on  the  south  side  1,720  yds.,  or 
a  total  of  4,235  yds.  out  of  21,575  yds.,  the  full  length  of  the 
tunnel.  Allowing  for  unavoidable  and  unforeseen  occurrences, 
such  as  strikes,  war,  etc.,  the  contractors  expect  to  complete 
tunnel  No.  1  and  the  parallel  heading  by  May,  1904. 


114  TUNNELING 


CHAPTER   XL 

TUNNELS   THROUGH    HARD    ROCK    (Continued).— 

EXCAVATION    BY   DRIFTS.  — ST.    GOTHARD 

TUNNEL. —BUSK   TUNNEL. 


THE  more  common  method  of  tunneling  through  hard  rock 
is  to  begin  the  work  by  a  heading,  instead  of  by  a  drift.  This 
heading  may  be  of  small  dimensions,  and  the  remainder  of  the 
section  may  also  be  removed  in  successive  small  parts,  or  it  may 
be  the  full  width  of  the  section,  and  the  enlargement  of  the 
section  be  made  in  one  other  cut. 

General  Discussion.  —  When  the  tunnel  is  excavated  by  means 
of  several  cuts,  which  is  the  method  usually  employed  in 
Europe,  the  sequence  of  work  is  as  indicated  by  Fig.  5*b 
Work  is  begun  by  driving  the  center  top  heading  No  1,  whose 
floor  is  at  the  level  of  the  bottom  of  the  roof  arch,  and  which  is 
usually  excavated  by  the  circular  cut  method.  This  heading  is 
widened  by  removing  parts  No.  2  until  the  top  part  of  the  sec- 
tion is  removed,  when  the  roof  arch  is  built  with  its  feet  rest- 
ing on  the  unexcavated  rock  below.  The  lower  portion  of  the 
section  or  bench  is  removed  by  first  sinking  the  trench  No.  3, 
after  which  part  No.  4  is  taken  out,  and  then  part  No.  5,  and 
the  side  walls  built.  Part  No.  6  for  the  culvert  is  finally 
opened.  The  heading  is,  as  a  rule,  driven  far  in  advance,  but 
the  excavation  of  each  of  the  other  parts  follows  the  preceding 
one  at  a  distance  behind  of  about  300  ft. 

The  strutting,  when  any  is  required,  is  usually  the  typical 
radial  strutting  of  the  Belgian  method  of  tunneling.  The 
masonry  lining  is  constructed  practically  the  same  as  in  tunnels 
excavated  by  a  drift.  The  hauling  is  done  on  a  single  track 
laid  in  the  heading  No.  1,  which  separates  into  double  tracks 


TUNNELS   THROUGH    HARD    ROCK  115 

where  the  full  top  section  has  been  excavated  by  the  removal 
of  parts  No.  2.  These  two  tracks  are  again  combined  and  form 
a  single  track  along  the  top  of  part  No.  5,  which  has  been  left 
wider  than  part  No.  4  for  this  particular  purpose.  When»part 
No.  3  is  excavated  a  standard-gauge  track  is  laid  on  its  floor ; 
and  as  the  full  section  of  the  tunnel  is  completed  by  taking  out 
parts  Nos.  4  and  5,  this  single  track  is  replaced  by  two  standard- 
gauge  tracks,  into  which  it  switches.  Spoil  is  transferred  from 
the  narrow-gauge  tracks  on  the  upper  level,  to  the  standard- 
gauge  tracks  on  the  tunnel  floor,  by  means  of  chutes,  and  build- 
ing material  is  transferred  in  the  opposite  direction  by  means  of 
hoisting  apparatus. 

When  the  excavation  is  made  by  a  single  wide  heading,  and 
a  single  other  cut  for  removing  the  bench,  which  is  the  method 
preferred  by  American  engineers,  the  work  begins  by  removing 
a  top  heading  the  full  width  of  the  section.  This  heading  is 
usually  made  7  ft.  or  8  ft.  high,  and  is  excavated  by  the  center  cut 
method.  The  method  of  strutting  usually  employed,  is  to  erect 
successive  three-  or  five-segment  timber  arches,  whose  feet  rest 
on  the  top  of  the  bench ;  when  the  bench  is  removed,  posts  are 
inserted  under  the  feet  of  each  arch.  These  arches  are  covered 
with  a  lagging  of  plank.  In  America  it  has  often  been  the 
practice  to  let  this  strutting  serve  as  a  temporary  lining,  and  to 
replace  it  only  after  some  time,  often  after  years,  with  a  perma- 
nent lining  of  masonry.  In  a  succeeding  chapter,  some  of  the 
methods  adopted  in  relining  timber-lined  arches  with  masonry 
are  described.  The  hauling  is  done  by  a  narrow-gauge  track 
laid  on  the  bottom  of  the  heading,  and  by  either  narrow  or 
broad  gauge  tracks  laid  on  the  floor  of1  the  completed  section 
below.  A  device  called  a  bench  carriage  is  often  employed  to 
enable  the  cars  running  on  the  heading  tracks  to  dump  their 
loads  into  the  cars  below,  without  interfering  with  the  work  on 
the  bench  front.  This  device  consists  of  a  wide  platform 
carried  on  trucks,  running  on  rails  at  the  sides  of  the  tunnel 
floor,  so  that  it  is  level  with  the  floor  of  the  heading.  The 


116 


TUNNELING 


front  of  this  platform  carries  a  hinged  leaf  which  may  be  raised 
and  lowered,  and  which  forms  a  sort  of  gang-plank  reaching  to 
the  floor  of  the  heading.  By  running  the  heading  cars  out  on  to 
this  traveling  platform,  they  can  be  dumped  into  the  cars  below 
entirely  clear  of  the  work  in  progress  on  the  bench  front. 

For  the  purpose  of  illustrating  the  two  methods  of  driving 
tunnels  by  a  heading,  which  have  been  briefly  described,  the  St. 
Gothard  and  the  Busk  tunnels  have  been  selected.  The  St. 
Gothard  tunnel  is  selected,  as  being  the  longest  tunnel  in  the 
world,  and  because  it  was  excavated  by  a  number  of  small  parts ; 
and  the  Busk  tunnel,  as  being  a  single-track  tunnel,  driven  by 
a  heading,  and  bench,  and  having  a  timber  lining. 

St.  Gothard  Tunnel.  —  The  St.  Gothard  tunnel  penetrates  the 
Alps  between  Italy  and  France,  and  is  9^  miles  long.  It  was 
constructed  in  1872-82. 

Material  Penetrated, — The  St.  Gothard  tunnel  was  excavated 
through  rock,  consisting  chiefly  of  gneiss,  mica-schist,  serpen- 
tine, and  hornblend,  the  strata  having  an  inch' nation  of  from 
45°  to  90°.  At  many  points  the  rock  was  fissured,  and  disin- 
tegrated easily,  and  water  was  en- 
countered in  large  quantities,  caus- 
ing much  trouble. 

Excavation.  —  The  sequence  of 
excavation  is  shown  by  Fig.  15, 
p.  32.  First  the  top  center  head- 
ing, No.  1,  whose  dimensions  varied 
frqm  8.25  x  8.6  ft.  to  8.5  x  9  ft., 
according  to  the  quality  of  the  rockr 
was  driven  never  less  than  1,000  ft. 
and  sometimes  over  3,000  ft.  in 
advance  of  parts  No.  2.  The  exca- 
vation of  parts  No.  2  opened  up  the  full  top  section,  and  parts 
Nos.  3,  4,  5,  6,  and  7,  were  removed  in  the  order  numbered. 

Strutting.  —  Where  regular  strutting  was  required,  the  con- 
struction shown  in  Fig.  58  was  adopted. 


FIG.  58.  —  Diagram  Showing  Se- 
quence of  Excavation  in  Heading 
Method  of  Tunneling  Rock. 


TUNNELS   THROUGH    HARD   HOCK  11 7 

Masonry.  —  The  St.  Gothard  tunnel  is  lined  throughout  with 
masonry.  After  the  upper  portion  of  the  section  was  fully 
excavated,  the  roof  arch  was  built  with  its  feet  resting  upon 
short  planks  on  the  top  of  the  bench.  Plank  centers  were  used 
in  constructing  the  arch.  For  the  arch  brick  masonry  was 
employed,  but  the  side  walls  were  built  of  rubble  masonry. 
Shelter  niches,  about  3  ft.  deep,  were  built  into  the  side  walls 
at  intervals,  and  about  every  3,000  ft.  storage  niches  about  10 
ft.  deep,  and  closed  with  a  door,  were  constructed.  The  cul- 
vert was  of  brick  masonry. 

Mechanical  Installation.  —  Water-power  was  used  exclusively 
in  driving  the  St  Gothard  tunnel.  At  the  north  end,  the 
Reuss,  and  at  the  south  end,  the  Tessin  and  the  Tremola,  rivers 
or  torrents  were  dammed,  and  their  waters  conducted  to  tur- 
bine plants  at  the  opposite  ends  of  the  tunnel.  The  power  thus 
furnished  by  the  Reuss  was  about  1,500  H.P.,  and  the  power 
furnished  by  the  combined  supply  of  the  Tessin  and  Tremola 
was  1,220  H.P.  The  turbine  plant  at  both  ends  at  first  con- 
sisted of  four  horizontal  impulse  turbines,  but  later,  two  more 
turbines  were  added  at  the  south  end.  Each  of  the  two  sets  of 
four  turbines  first  installed  drove  five  groups  of  three  compres- 
sors each,  and  the  two  supplementary  turbines  drove  two  groups 
of  four  compressors  each.  The  compressors  were  of  the  Colladon 
type  with  water  injection,  and  four  groups  of  three  compressors 
each  were  capable  of  furnishing  1,000  cu.  yds.  of  air  compressed 
to  between  seven  and  eight  atmospheres  every  hour,  or  about 
100  H.P.  per  hour,  delivered  to  the  drills  at  the  front.  This 
air  when  exhausted  provided  about  8,000  cu.  yds.  of  fresh  air 
per  hour  for  ventilation. 

The  compressors  at  each  entrance  discharged  into  a  group 
of  four  cylindrical  receivers  of  wrought-iron  each  5.3  ft.  in 
diameter  by  29.5  ft.  long,  and  having  a  capacity  of  593  cu.  ft. 
The  cylinders  were  placed  horizontally,  the  first  one  receiving 
the  air  at  one  end  and  discharging  it  at  the  other  end  into  the 
next  cylinder,  and  so  on.  By  this  arrangement  the  air  was 


118  TUNNELING 

drained  of  its  moisture,  and  the  discharge  from  the  end  receiver 
into  the  tunnel  delivery  pipes  was  not  affected  by  the  pulsations 
of  the  compressors.  The  delivery  pipe  decreased  from  8  in. 
in  diameter  at  the  receiver  to  4  ins.  in  diameter,  and  finally  to 
2^  ins.  in  diameter,  at  the  front. 

The  drills  employed  were  of  various  patterns.  The  first  one 
employed  was  the  Dubois  &  Frangois  "  perforator,"  in  which  the 
drill-bit  was  fed  forward  by  hand.  This  was  replaced  by  Fer- 
roux  drills  having  an  automatic  feed.  Jules  McKean's  "  perfo- 
rator "  was  employed  at  the  north  end  of  the  tunnel.  All  of 
these  drills  were  of  the  percussion  type,  and  were  mounted  on 
carriages  running  on  tracks.  Their  comparative  efficiency  was 
officially  tested  in  drilling  granitic  gneiss  with  an  operating 
air  pressure  of  5.5  atmospheres  with  the  following  results : 

NAME  OF  DRILL.  PENETRATION  INS.  PER  MIN. 

Ferroux       .          1.6 

McKean 1.4 

Dubois  &  Frangois 1.04 

Souinmelier 0.85 

The  heading  was  excavated  by  the  circular  cut  method,  the 
holes  being  driven  as  follows :  Near  the  center  of  the  heading 
three  holes  were  first  drilled,  converging  so  as  to  inclose  a 
pyramid  with  a  triangular  base.  Around  these  center  holes 
from  9  to  13  others  were  driven  parallel  to  the  tunnel  axis. 
The  center  holes  were  blasted  first,  and  then  the  surrounding 
holes.  From  3  to  5  hours  were  required  to  drill  the  two  sets 
of  holes,  and  from  three  to  four  hours  were  required  to  remove 
the  blasted  rock.  The  number  of  holes  drilled  in  removing 
each  of  the  various  parts  was  as  follows : 

Part  No.  1 6  to  9 

Part  No.  2 6  to  10 

Part  No.  3 2 

Part  No.  4 6  to  9 

Part  No.  5 3 

Part  No.  6 6  to  9 

Part  No.  7  .  1 


Total  for  full  section 36  to  40 


TUNNELS  THROUGH  HARD  ROCK 


119 


Hauling.  —  Two  different  systems  were  employed  for  haul- 
ing the  spoil  and  construction  material  in  the  St.  Gothard 
tunnel.  To  remove  the  spoil  from  parts  Nos.  1  and  2  a  narrow- 
gauge  track  was  laid  on  the  floor  of  the  heading,  and  tLe  cars 
were  hauled  by  horses,  the  grade  being  descending  from  the 
fronts.  These  narrow-gauge  cars  were  dumped  into  larger 
broad-gauge  cars  running  on  the  track  laid  on  the  floor  of  the 
completed  section  and  hauled  by  compressed  air  locomotives 
(Fig.  59).  To  raise  the  incoming  structural  material  from  the 
broad-gauge  cars  to  the  narrow-gauge  cars  running  on  the  level 
above,  hoisting  devices  were  employed. 


Method  of  Strutting  Roof,  St  Gothard 
Tunnel. 


Sketch  Showing  Arrangement  of  Car 
Tracks,  St.  Gothanl  Tunnel. 


FIG.  59. 


Busk  Tunnel.  —  The  Busk  tunnel,  9,094  ft  long,  was  built 
between  Busk  and  Ivanhoe  stations,  on  the  Colorado  Midland 
R.R.  in  Colorado.  Fig.  60  is  a  trans  verse  section  of  the 
tunnel ;  it  is  for  a  single  track,  and  is  15  ft.  wide  and  21  ft. 
high. 

Material  Penetrated.  —  The  material  through  which  the 
tunnel  was  driven  was  a  gray  granite  of  irregular  character. 
In  some  places  the  rock  was  found  extremely  hard  to  drill  and 
blast,  and  stood  perfectly  upon  exposure  to  the  air,  while  in  other 
places,  where  it  seemed  at  first  equally  as  hard  and  firm,  it  dis- 
integrated upon  exposure,  and  it  was  found  necessary  to  timber 


120 


TUNNELING 


the  excavation.  In  other 
places,  where  no  disinte- 
gration was  apparent, 
the  rock  was  full  of 
seams  and  faults,  and  it 
was  necessary  to  support 
the  detached  fragments 
by  timbering.  In  a  few 
places  quite  large  cavi- 
ties were  encountered, 
which  were  filled  with 
liquid  mud.  In  one  place 
the  inrush  of  liquid  mud 
was  so  sudden  and  the 
stream  so  strong  that 
the  men  barely  escaped 
with  their  lives. 
Excavation.  —  The  excavation  was  made  by  a  heading  7  ft. 
high  and  the  full  width  of  the  section,  and  by  a  single  bench 
excavation.  -  In  driving  the  heading  two  sets  of  holes  were 


OouWe  Timtering  in  Heavy  (around. 

FIG.  60.  —  Transverse  Section  of  Busk  Tunnel  Colorado 
Midland  R.  R.,  Colorado. 


TUNNELS    THROUGH    HARD   ROCK  121 

drilled.  The  first  set  of  eight  holes  were  driven  in  two  rows 
from  top  to  bottom,  the  holes  being  about  2  ft.  apart  on  the 
surface,  and  converging  toward  the  center  of  the  t^innel. 
These  holes  were  12  ft.  deep,  and  the  action  of  the  blast  was 
to  blow  out  a  wedge-shaped  cavity  in  the  face.  The  holes  of 
the  second  set  were  drilled  at  the  sides  of  the  front  and 
parallel  to  the  sides  of  the  section,  and  the  blast  blew  out  the 
remainder  of  the  rock  into  the  wedge-shaped  center  cavity. 
The  method  of  excavating  the  bench  was  nearly  the  same  as 
that  of  excavating  the  heading. 

Mechanical  Installation.  —  The  following  machinery  was 
employed  in  connection  with  the  construction  of  the  tunnel: 
at  the  Ivanhoe  end,  three  100  H.  P.  boilers;  two  20  x  24  in. 
Ingersoll  compressors,  and  one  20  X  24  in.  Norwalk  compres- 
sor ;  a  10  H.  P.  engine  driving  an  electric-light  dynamo,  and  a 
20  H.  P.  engine  driving  a  No.  6  Baker  blower,  forcing  fresh 
air  into  the  tunnel  through  a  14-in.  pipe.  In  the  tunnel  one 
No.  7  and  one  No.  9  Cameron  pump,  and  a  Deane  duplex 
pump  with  a  10-in.  stroke,  were  employed  to  keep  the  excava- 
tion clear  of  water,  since  ths  grade  descended  uniformly  from 
the  Ivanhoe  end,  and  the  water  followed  the  workings.  At  the 
Busk  end  the  plant  consisted  of  three  80  H.  P.  boilers,  two 
20  x  24-in.  Ingersoll  compressors,  10  H.  P.  and  20  H.  P.  en- 
gines respectively,  for  the  electric  light  dynamo  and  the  blower. 
Four  3^  in.  Ingersoll  eclipse  drills  were  used  in  each  heading, 
and  two  on  each  bench,  making  six  drills  at  each  end  of  the 
tunnel. 

Strutting  and  Lining.  —  For  about  78  %  of  its  length  the 
tunnel  is  lined  with  timber.  The  timbering  consists  of  a  five- 
segment  arch  for  the  roof,  resting  on  a  wall  plate  which  is  car- 
ried by  vertical  side  posts.  The  segments  of  the  arch,  the  wall 
plates,  and  the  posts,  are  12x1 2-inch  timbers.  The  roof  arches 
and  the  posts  supporting  the  wall  plates  are  spaced  4  ft.  apart, 
center  to  center.  Above  the  arches  is  laid  a  lagging  of  2-inch 
longitudinal  planks.  The  arches  were  set  up  as  fast  as  the 


122  TUNNELING 

heading  was  driven,  and  rested  upon  the  bench  until  it  was 
removed  and  the  side  posts  inserted.  Where  mud  pockets 
were  met  the  plank  lagging  was  inserted  behind  the  side  posts 
as  well  as  above  the  roof-arch  ribs,  and  when  the  pressures  were 
unusually  great  a  double  lining  was  employed. 

Progress  of  Work.  —  The  rate  of  progress  made  in  exca- 
vating the  Busk  tunnel  was  as  follows :  — 

Total  time  consumed  in  driving  the  heading 1,118  days 

Average  daily  progress  for  both  headings 8.4  feet 

Greatest  progress  in  one  month 337    " 

Average  daily  progress,  one  month,  31  days 10.87     u 

Greatest  progress  in  one  month  (28  days)  at  one  end    .     .  202.5    " 

Average  progress  in  one  month  (28  days)  at  one  end     .     .  7.23     " 

Greatest  monthly  progress  on  bench 218    " 

Average  daily  progress  on  bench        7.79  " 

Cost  of  Work.  —  The  cost  of  the  tunnel  was  calculated  on 
the  assumption  that  the  excavation  per  lineal  foot  was  10.19 
cu.  yds.,  and  where  the  section  was  enlarged  for  timbering, 
1379  cu.  yds.  The  contractors'  estimate  for  excavating  and 
timbering  the  tunnel  was  as  follows  :  — 

Excavation  of  9,393.66  lineal  feet  @  $62,50        ....  $587,103.73 

Enlargement  for  timbering  32,575  cubic  yards        .     .     .  81,437.50 

Cost  of  timber 81,600.00 

Cost  of  labor  on  timbering  2,723,000  ft.  B.  M.  @  $12      .  32,676.00 

Total 8782,817.25 

This  is  an  average  cost  per  lineal  foot  of  tunnel  of  $83.14,, 
which  is  very  close  to  the  average  cost  of  single-track  timber- 
lined  tunnels  in  America,  which  is  usually  figured  at  $85  per 
lineal  foot. 

COMPARISON  OF    METHODS. 

The  differences  between  the  drift  and  heading  methods  of 
excavating  tunnels  through  rock,  consist  chiefly  in  the  excava- 
tions, strutting,  and  hauling.  When  the  drift  method  is  em- 
ployed an  advanced  gallery  is  opened  along  the  floor  of  the 


TUNNELS    THROUGH    HARD    ROCK  12& 

tunnel  before  the  upper  part  of  the  section  is  removed,  and 
when  the  heading  method  is  employed  the  upper  part  of  the 
section  is  completely  excavated  and  lined  before  any  part  of 
the  section  below  is  excavated.  When  the  drift  method  of 
driving  is  employed  polygonal  strutting  is  usually  used,  and 
longitudinal  strutting  is  employed  with  the  heading  method  of 
driving.  In  the  drift  method  the  hauling  is  done  by  one  system 
of  tracks  at  the  same  level,  while  in  the  heading  method  two 
systems  of  tracks  are  employed  at  different  levels. 

It  is,  perhaps,  impossible  to  state  without  qualification,  which 
method  is  the  better.  European  engineers  unanimously  prefer 
excavation  by  a  drift,  especially  for  long  tunnels.  An  advan- 
tage that  this  method  affords  in  long  tunnels  is,  that  the  water 
which  is  usually  found  in  large  quantities  under  high  moun- 
tains is  easily  collected  in  the  drift  and  conveyed  to  the  culvert, 
while  in  the  heading  method  the  water  from  the  advance  gallery 
before  being  collected  into  the  culvert  built  on  the  floor  of  the 
tunnel,  must  pass  through  all  the  workings.  This  may  be  a 
serious  inconvenience  when  water  is  found  in  large  quantities, 
as,  for  instance,  was  the  case  in  the  St  Gothard  tunnel,  where 
the  stream  amounted  to  57  gallons  per  second.  The  heading 
method  has  an  advantage  in  tunneling  loose  rock,  since  it  is  the 
more  economical  in  strutting. 


TUNNELING 


CHAPTER   XII. 

REPRESENTATIVE      MECHANICAL       INSTALLA- 
TIONS   FOR    TUNNEL    WORK. 


THE  important  role  played  by  the  power  plant  and  other 
mechanical  installations  in  constructing  tunnels  through  rock 
has  already  been  mentioned.  In  some  methods  of  soft-ground 
tunneling,  and  particularly  in  soft-ground  subaqueous  tunnel- 
ing, it  is  also  often  necessary  to  employ  a  mechanical  installa- 
tion but  slightly  inferior  in  size  and  cost  to  those  used  in 
tunneling  rock.  The  general  character  of  the  mechanical 
plant  required  for  tunnel  work  has  been  described  in  another 
chapter.  It  is  proposed  to  describe  very  briefly  here  a  few 
typical  individual  plants  of  this  character,  which  will  in  some 
respects  give  a  better  idea  of  this  phase  of  tunnel  work  than 
the  more  general  descriptions. 

Rock  Tunnels.  —  The  tunnels  selected  to  illustrate  the  me- 
chanical installations  employed  in  tunneling  through  rock  ore : 
The  Hoosac  Tunnel,  the  Cascade  Tunnel,  the  Niagara  Falls 
Power  Tunnel,  the  Palisades  Tunnel,  the  Croton  Aqueduct 
Tunnel,  the  Strickler  Tunnel,  in  America,  and  the  Graveholz 
Tunnel  and  the  Sonnstein  Tunnel  in  Europe.  In  addition 
there  will  be  found  in  other  chapters  of  this  book  a  description 
of  the  mechanical  installation  at  the  Busk  tunnel  and  at  the 
St.  Gothard  and  Simplon  tunnels. 

Hoosac  Tunnel.  —  The  Hoosac  tunnel  on  the  Fitchburg  R.R. 
in  Massachusetts  is  25,000  ft.  long,  and  the  longest  tunnel  in 
America.  The  material  through  which  the  tunnel  was  driven 
was  chiefly  hard  granitic  gneiss,  conglomerate,  and  mica-schist 
rock.  The  excavation  was  conducted  from  the  entrances  and 


MECHANICAL   INSTALLATIONS   FOR    TU 

one  shaft,  the  wide  heading  and  single-beii^k«^tKod  being 
employed,  with  the  center-cut  system  of  blasting  which  was 
here  used  for  the  first  time.  The  tunnel  was  begun  in  1854, 
and.  continued  by  hand  until  1866,  when  the  mechanical  plant 
was  installed.  Most  of  the  particular  machines  employed  have 
now  become  obsolete,  but  as  they  were  the  first  machines  used 
for  rock  tunneling  in  America  they  deserve  mention.  The 
drills  used  were  Burleigh  percussion  drills,  operated  by  com- 
pressed air.  Six  of  these  drills  were  mounted  on  a  single  car- 
riage, and  two  carriages  were  used  at  each  front.  The  air  to 
operate  these  drills  was  supplied  by  air  compressors  operated 
by  water-power  at  the  portals  and  steam-power  at  the  shaft. 
The  air  compressors  consisted  of  four  horizontal  single-acting 
air  cylinders  with  poppet  valves  and  water  injection.  The 
compressors  were  designed  by  Mr.  Thomas  Deane  the  chief 
engineer  of  the  tunnel. 

Palisades  Tunnel.  —  The  Palisades  tunnel  was  constructed  to 
carry  a  double-track  railway  line  through  the  ridge  of  rocks 
bordering  the  west  bank  of  the  Hudson  River  and  known  as 
the  Palisades.  It  was  located  about  opposite  116th  St.  in  New 
York  city.  The  material  penetrated  was  a  hard  trap  rock  very 
full  of  seams  in  places,  which  caused  large  fragments  to  fall 
from  the  roof.  The  excavation  was  made  by  a  single  wide 
heading  and  bench,  employing  the  center-cut  method  of  blast- 
ing with  eight  center  holes  and  16  side  holes  for  the  7  x  18  ft. 
heading.  Ingersoll-Sergeant  2^  in.  drills  were  used,  four  in 
each  heading  and  six  on  each  bench,  and  30  ft.  per  10  hours 
was  considered  good  work  for  one  drill. 

The  power-plant  was  situated  at  the  west  portal  of  the 
tunnel,  and  the  power  was  transmitted  by  electricity  and  com- 
pressed air  to  the  middle  shaft  and  east  portal  workings.  The 
plant  consisted  of  eight  100  H.  P.  boilers,  furnishing  steam  to 
four  Rand  duplex  18  X  22  in.  air  compressors,  and  an  engine 
running  a  30  arc  light  dynamo.  The  compressed  air  was  car- 
ried over  the  ridge  by  pipes  varying  from  10  ins.  to  5  ins.  in 


126  TUNNELING 

diameter  to  the  shaft  and  to  the  east  portal,  and  was  used  for 
operating  the  hoisting  engines  as  well  as  the  drills  at  these 
workings.  Inside  the  tunnel,  specially  designed  derrick  cars 
were  employed  to  handle  large  stones,  they  being  also  operated 
by  compressed  air.  This  car  ran  on  a  center  track,  while  the 
mucking  cars  ran  on  side  tracks,  and  it  was  employed  to  lift 
the  bodies  of  the  cars  from  the  trucks,  place  them  close  to  the 
front,  being  worked  where  large  stone  could  be  rolled  into 
them,  and  return  them  to  the  trucks  for  removal.  In  addition 
to  handling  the  car  bodies  the  derrick  was  used  to  lift  heavy 
stones.  The  hauling  was  done  at  first  by  horse-power,  and 
later  by  dummy  locomotives. 

Croton  Aqueduct  Tunnel.  —  In  the  construction  of  the  Croton 
Aqueduct  for  the  water  supply  of  New  York  city,  a  tunnel  31 
miles  long  was  built,  running  from  the  Croton  Dam  to  the 
Gate  House  at  135th  St.  in  New  York  city.  The  section  of 
the  tunnel  varies  in  form,  but  is  generally  either  a  circular  or  a 
horse-shoe  section.  In  all  cases  the  section  was  designed  to 
have  a  capacity  for  the  flow  of  water  equal  to  a  cylinder  14  ft. 
in  diameter.  To  drive  the  tunnel,  40  shafts  were  employed. 
The  material  penetrated  was  of  almost  every  character,  from 
quicksand  to  granitic  rock,  but  the  bulk  of  the  work  was  in 
rock  of  some  character.  The  excavation  in  rock  was  conducted 
by  the  wide  heading  and  bench  method,  employing  the  center- 
cut  method  of  blasting.  Four  air  drills,  mounted  on  two 
double-arm  columns,  were  employed  in  the  heading.  The 
drills  for  the  bench  work  were  mounted  on  tripods.  Steam- 
power  was  used  exclusively  for  operating  the  compressors, 
hoisting  engines,  ventilating  fans  and  pumps ;  but  the  size  and 
kind  of  boilers  used,  as  well  as  the  kind  and  capacity  of  the 
machines  which  they  operated,  varied  greatly,  since  a  separate 
power-plant  was  employed  for  each  shaft  with  a  few  exceptions. 
A  description  of  the  plant  at  one  of  the  shafts  will  give  an 
indication  of  the  size  and  character  of  those  at  the  other  shafts, 
and  for  this  purpose  the  plant  at  shaft  10  has  been  selected. 


MECHANICAL   INSTALLATIONS    FOR    TUNNEL   WORK      127 

At  shaft  10  steam  was  provided  by  two  Ingersoll  boilers  of 
80  H.  P.  each,  and  by  a  small  upright  boiler  of  8  H.  P.  There 
were  two  18  X  30  in.  Ingersoll  air  compressors  pumping  into 
two  42  in.  X  10  ft.  and  two  42  in.  X  12  ft.  Ingersoll  receivers. 
In  the  excavation  there  were  twelve  3£  in.  and  six  3|  in. 
Ingersoll  drills,  four  drills  mounted  on  two  double-arm  columns 
being  used  on  each  heading,  and  the  remainder  mounted  on 
tripods  being  used  on  the  bench.  Two  Dickson  cages  operated 
by  one  12  x  12  in.  Dickson  reversible  double  hoisting  engine 
provided  transportation  for  material  and  supplies  up  and  down 
the  shaft.  A  Thomson-Houston  ten-light  dynamo  operated  by 
a  Lidgerwood  engine  provided  light.  Drainage  was  effected  by 
means  of  two  No.  9  and  one  No.  6  Cameron  pumps.  At  this 
particular  shaft  the  air  exhausted  from  the  drills  gave  ample 
ventilation,  especially  when  after  each  blast  the  smoke  was 
cleared  away  by  a  jet  of  compressed  air.  In  other  workings, 
however,  where  this  means  of  ventilation  was  not  sufficient, 
Baker  blowers  were  generally  employed. 

Strickler  Tunnel. —-The  Strickler  tunnel  for  the  water 
supply  of  Colorado  Springs,  Col.,  is  6,441  ft.  long  with  a  sec- 
tion of  4  ft.  x  7  ft.  It  penetrates  the  ridge  connecting  Pike's 
Peak  and  the  Big  Horn  Mountains,  at  an  elevation  of  11,540 
ft.  above  sea  level.  The  material  penetrated  is  a  coarse 
porphyritic  granite  and  morainal  debris,  the  portion  through 
the  latter  material  being  lined.  The  mechanical  installation 
consisted  of  a  water-power  electric  plant  operating  air  com- 
pressors. The  water  from  Buxton  Creek  having  a  fall  of 
5,400  ft.  was  utilized  to  operate  a  36  in.  220  H.  P.  Pelton 
water-wheel,  which  operated  a  150  K.  W.  three-phase  generator. 
From  this  generator  a  3,500  volt  current  was  transmitted  to 
the  east  portal  of  the  tunnel,  where  a  step-down  transformer 
reduced  it  to  a  220  volt  current  to  the  motor.  The  transmis- 
sion line  consisted  of  three  No.  5  wires  earned  on  cross-arm 
poles  and  provided  with  lightning  arresters  at  intervals.  The 
plant  at  the  east  portal  of  the  tunnel  consisted  of  a  75  H.  P. 


128  TUNNELING 

electric  motor,  driving  a  75  H.P.  air  compressor,  and  of  small 
motors  to  drive  a  Sturtevant  blower  for  ventilation,  to  run  the 
blacksmith  shop,  and  to  light  the  tunnel,  shop,  and  yards. 
From  the  compressor  air  was  piped  into  the  tunnel  at  the 
east  end,  and  also  over  the  mountain  to  the  west  portal  work- 
ings. Two  drills  were  used  at  each  end,  and  the  air  wa& 
also  used  for  operating  derricks  and  other  machinery.  For 
removing  the  spoil  a  trolley  carrier  system  was  employed.  A 
longitudinal  timber  was  fastened  to  the  tunnel  roof,  directly 
in  the  apex  of  the  roof  arch.  This  timber  carried  by  means- 
of  hangers  a  steel  bar  trolley  rail  on  which  the  carriages  ran. 
Outside  of  the  portal  this  rail  formed  a  loop,  so  that  the 
carriage  oould  pass  around  the  loop  and  be  taken  back  to- 
the  working  face.  Each  carriage  carried  a  steel  span  of  1^  cu. 
ft.  capacity,  so  suspended  that  by  means  of  a  tripping  device 
it  was  automatically  dumped  when  the  proper  point  on  the 
loop  was  reached. 

Niagara  Falls  Power  ,  annel.  —  The  tail-race  tunnel  built 
to  carry  away  the  water  discharged  from  the  turbines  of  the 
Niagara  Falls  Power  Co.,  has  a  horse-shoe  section  19  x  21  ft. 
and  a  length  of  6,700  ft.  It  was  driven  through  rock  from 
three  shafts  by  the  center-cut  method  of  blasting.  In  sink- 
ing shaft  No.  0  very  little  water  was  encountered,  but  at  shafts- 
Nos.  1  and  2  an  inflow  of  800  gallons  and  600  gallons  per 
minute,  respectively,  was  encountered.  The  principal  plant 
was  located  at  shaft  No.  2,  and  consisted  of  eight  100  H.  P. 
boilers,  three  18  x  30  in.  Rand  duplex  air  compressors,  a 
Thomson-Houston  electric-light  plant,  and  a  sawmill  with  a 
capacity  of  20,000  ft.  B.  M.  per  day.  The  shafts  were  fitted 
with  Otis  automatic  hoisting  engines,  with  double  cages  at 
shafts  Nos.  1  and  2,  and  a  single  cage  at  shaft  No.  0.  The 
drills  used  were  25  Rand  drills  and  three  Inge rsoll- Sergeant 
drills.  The  pumping  plant  at  shaft  No.  2  consisted  of  four 
No.  7  and  ono  Cameron  pumps,  and  that  at  shaft  No.  2 

consisted  of  two   N,  -.  7  and  two  No.  9  Cameron  pumps  and 


MECHANICAL  INSTALLATIONS   FOR   TUNNEL   WORK      129 

three  Snow  pumps.  An  auxiliary  boiler  plant  consisting  of 
two  60  H.  P.  boilers  was  located  at  shaft  No.  1,  and  another, 
consisting  of  one  75  H.  P.  boiler,  was  located  at  shaft  No*  0. 

Cascade  Tunnel.  —  The  Cascade  tunnel  was  built  in  1886- 
88  to  carry  the  double  tracks  of  the  Northern  Pacific  Ry. 
through  the  Cascade  Mountains  in  Washington.  It  is  9,850  ft. 
long  with  a  cross-section  16i  ft.  wide  and  22  ft.  high,  and 
is  lined  with  masonry.  The  material  penetrated  was  a  basaltic 
rock,  with  a  dip  of  the  strata  of  about  5°.  The  rock  was 
excavated  by  a  wide  heading  and  one  bench,  using  the  center- 
cut  system  of  blasting.  A  strutting  consisting  of  five-segment 
timber  arches  carried  on  side  posts,  spaced  from  2  ft.  to  4  ft. 
apart,  and  having  a  roof  lagging  of  4  X  6  in.  timbers  packed 
above  with  cord- wood.  The  mechanical  plant  of  the  tunnel  is 
of  particular  interest,  because  of  the  fact  that  all  the  machinery 
and  supplies  had  to  be  hauled  from  82  to  87  miles  by  teams, 
over  a  road  cut  through  the  forests  covering  the  mountain 
slopes.  This  work  required  from  Feb.  22  to  July  15,  1886,  to 
perform.  In  many  places  the  grades  were  so  steep  that  the 
wagons  had  to  be  hauled  by  block  and  tackle.  The  plant  con- 
sisted of  five  engines,  two  water-wheels,  five  air  compressors, 
eight  70  H.  P.  steam-boilers,  four  large  exhaust  fans,  two  com- 
plete electric  arc-lighting  plants,  two  fully  equipped  machine- 
shop  outfits,  36  air  drills,  two  locomotives,  60  dump  care,  and 
two  sawmill  outfits,  with  the  necessary  accessories  for  these  vari- 
ous machines.  This  plant  was  divided  about  equally  between 
the  two  ends  of  the  tunnel.  The  cost  of  the  plant  and  of 
the  work  of  getting  it  into  position  was  $125,000. 

G-raveholz  Tunnel.  —  The  Graveholz  tunnel  on  the  Bergen 
Railway  in  Norway  is  notable  as  being  the  longest  tunnel  in 
northern  Europe,  and  also  as  being  built  for  a  single-track 
narrow-gauge  railway.  This  tunnel  is  17,400  feet  long,  and  is 
located  at  an  elevation  of  2,900  ft.  above  sea-level.  Only 
about  3  %  of  the  length  of  the  tunnel  is  lined.  The  mechani- 
cal installation  consists  of  a  turbine  plant  operating  the  various 


130  TUNNELING 

machines.  There  are  two  turbines  of  100  H.  P.  and  120  H.  P. 
taking  water  from  a  reservoir  on  the  mountain  slope,  and 
furnishing  220  H.  P.,  which  is  distributed  about  as  follows : 
Boring-machines,  60  H.  P. ;  ventilation,  30  to  40  H.  P. ;  elec- 
tric locomotives,  15  H.  P. ;  machine  shop,  15  H.  P. ;  electric- 
lighting  dynamo,  25  H.  P. ;  electric  drills,  the  surplus,  or  some 
40  H.  P.  The  boring-machines  and  electric  drills  will  be 
operated  by  the  smaller  100  H.  P.  turbine. 

Sonmtein  Tunnel.  —  -  The  Sonnstein  tunnel  in  Germany  is 
particularly  interesting  because  of  the  exclusive  use  of  Brandt 
rotary  drills.  The  tunnel  was  driven  through  dolomite  and 
hard  limestone  by  means  of  a  drift  and  two  side  galleries.  The 
dimensions  of  the  drift  were  7i  X  7^  ft.  The  power  plant  con- 
sisted of  two  steam  pressure  pumps,  one  accumulator,  and  four 
drills.  The  steam-boiler  plant,  in  addition  to  operating  the 
pumps,  also  supplied  power  for  operating  a  rotary  pump  for 
drainage  and  a  blower  for  ventilation.  The  hydraulic  pressure 
required  was  75  atmospheres  in  the  dolomite,  and  from  85  to 
100  atmospheres  in  the  limestone.  The  drift  was  excavated 
with  five  3^  in.  holes,  one  being  placed  at  the  center  and 
driven  parallel  to  the  axis  of  the  tunnel,  and  four  being  placed 
at  the  corners  of  a  rectangle  corresponding  to  the  sides  of  the 
drift,  and  driven  at  an  angle  diverging  from  the  center  hole. 
The  average  depths  of  the  holes  were  4.3  ft.,  and  the  efficiency 
of  the  drills  was  1  in.  per  minute.  One  drill  was  employed 
at  each  front,  and  was  operated  by  a  machinist  and  two  helpers, 
who  worked  eight-hour  shifts,  with  a  blast  between  shifts  at 
first,  and  later  twelve-hour  shifts,  with  a  blast  between  shifts. 
The  24  hours  of  the  two  shifts  were  divided  as  follows :  boring 
the  holes,  10.7  hours ;  charging  the  holes,  1.1  hours ;  removing 
the  spoil,  11.7  hours;  changing  shifts,  0.5  hour.  The  average 
progress  per  day  for  each  machine  was  6.7  ft.  The  total  cost 
of  the  plant  was  $17,450. 

St.  Clair  River  Tunnel.  —  The  submarine  double-track  rail- 
way tunnel  under  the  St.  Clair  River  for  the  Grand  Trunk  Ry., 


MECHANICAL   INSTALLATIONS    FOR    TUNNEL    WORK       131 

is  8,500  ft.  long,  and  was  driven  through  clay  by  means  of  a 
shield,  as  described  in  the  succeeding  chapter  on  the  shield 
system  of  tunneling.  The  mechanical  plant  installed  for  pros- 
ecuting the  work  was  very  complete.  To  furnish  steam  to  the 
air  compressors,  pumps,  electric-light  engines,  hoisting-engines, 
etc.,  a  steam-plant  was  provided  on  each  side  of  the  river,  con- 
sisting of  three  70  H.  P.  and  four  80  H.  P.  Scotch  portable 
boilers.  The  air-compressor  plant  at  each  end  consisted  of 
two  20  X  24  in.  Ingersoll  air  compressors.  To  furnish  light  to 
the  workings,  two  100  candle-power  Edison  dynamos  were  in- 
stalled oft  the  American  side,  and  two  Ball  dynamos  of  the  same 
size  were  installed  on  the  Canadian  side.  The  dynamos  on 
both  sides  were  driven  by  Armington  &  Sims  engines.  These 
dynamos  furnished  light  to  the  tunnel  workings  and  to  the 
machine-shops  and  power-plant  at  each  end.  Root  blowers  of 
10,000  cu.  ft.  per  minute  capacity  provided  ventilation.  The 
pumping  plant  consisted  of  one  set  of  pumps  installed  for  per- 
manent drainage,  and  another  set  installed  for  drainage  during 
construction,  and  also  to  remain  in  place  as  apart  of  the  permanent 
plant.  The  latter  set  consisted  of  two  500  gallon  Worthington 
duplex  pumps  set  first  outside  of  each  air  lock,  closing  the  ends 
of  the  river  portion  of  the  tunnel.  For  permanent  drainage, 
a  drainage  shaft  was  sunk  on  the  Canadian  side  of  the  river, 
and  connected  with  a  pump  at  the  bottom  of  the  open-cut 
approach.  In  this  shaft  were  placed  a  vertical,  direct  acting, 
compound  condensing  pumping  engine  with  two  19J  in.  high- 
pressure  and  two  33|  in.  low-pressure  cylinders  of  24  in.  stroke, 
connected  to  double-acting  pumps  with  a  capacity  of  3000 
gallons  per  minute,  and  also  two  duplex  pumps  of  500  gallons 
capacity  per  minute.  For  permanent  drainage  on  the  American 
side,  four  Worthington  pumps  of  3,000  gallons'  capacity  were 
installed  in  a  pump-house  set  back  into  the  slope  of  the  open- 
cut  approach.  For  the  permanent  drainage  of  the  tunnel 
proper  two  400  gallon  pumps  were  placed  at  the  lowest  point 
of  the  tunnel  grade.  Spoil  coming  from  the  tunnel  proper  was 


132  TUNNELING 

hoisted  to  the  top  of  the  open  cut  by  derricks  operated  by  two 
50  H.  P.  Lidgerwood  hoisting-engines.  The  pressure  pumping 
plant  for  supplying  water  to  the  hydraulic  shield-jacks  at  each 
end  of  the  tunnel  consisted  of  duplex  direct-acting  engines 
with  12  in.  steam  cylinders  and  1  in.  water  cylinders,  supply- 
ing water  at  a  pressure  of  2000  Ibs.  per  sq.  in. 


TUNNELS  THROUGH  SOFT  GROUND         133 


CHAPTER   XIII. 

EXCAVATING     TUNNELS    THROUGH    SOFT 

GROUND  ;  GENERAL    DISCUSSION  ;  THE 

BELGIAN    METHOD. 


GENERAL  DISCUSSION. 

IT  may  be  set  down  as  a  general  truth  that  the  excavation 
of  tunnels  through  soft  ground  is  the  most  difficult  task  which 
confronts  the  tunnel  engineer.  Under  the  general  term  of  soft 
ground,  however,  a  great  variety  of  materials  is  included,  be- 
ginning with  stratified  soft  rock  and  the  most  stable  sands  and 
clays,  and  ending  with  laminated  clay  of  the  worst  character. 
From  this  it  is  evident  that  certain  kinds  of  soft-ground 
tunneling  may  be  less  difficult  than  the  tunneling  of  rock, 
and  that  other  kinds  may  present  almost  insurmountable  dif- 
ficulties. Classing  both  the  easy  and  the  difficult  materials 
together,  however,  the  accuracy  of  the  statement  first  made 
holds  good  in  a  general  way.  Whatever  the  opinion  may  be 
in  regard  to  this  point,  however,  there  is  no  chance  for  dispute 
in  the  statement  that  the  difficulty  of  tunneling  the  softer  and 
more  treacherous  clays,  peats,  and  sands  is  greater  than  that 
of  tunneling  firm  soils  and  rock ;  and  if  we  describe  the  methods 
which  are  used  successfully  in  tunneling  very  unstable  materials, 
no  difficulty  need  be  experienced  in  modifying  them  to  handle 
stable  materials. 

Characteristics  of  Soft-Ground  Tunneling.  —  The  principal  char- 
acteristics which  distinguish  soft-ground  tunneling  are,  first, 
that  the  material  is  excavated  without  the  use  of  explosives, 
and  second,  that  the  excavation  has  to  be  strutted  practically 


134  TUNNELING 

as  fast  as  it  is  completed.  In  treacherous  soils  the  excavation 
also  presents  other  characteristic  phenomena:  The  material 
forming  the  walls  of  the  excavation  tends  to  cave  and  slide. 
This  tendency  may  develop  immediately  upon  excavation,  or  it 
may  be  of  slower  growth,  due  to  weathering  and  other  nat- 
ural causes.  In  either  case  the  roof  of  the  excavations  tends 
to  fall,  the  sides  tend  to  cave  inward  and  squeeze  together,  and 
the  bottom  tends  to  bulge  or  swell  upward.  In  materials  of 
very  unstable  character  these  movements  exert  enormous  pres- 
sures upon  the  timbering  or  strutting,  and  in  especially  bad 
cases  may  destroy  and  crush  the  strutting  completely.  Out- 
side the  tunnel  the  surface  of  the  ground  above  sinks  for  a  con- 
siderable distance  on  each  side  of  the  line  of  the  tunnel. 

Methods  of  Soft-Ground  Tunneling. — There  are  a  variety  of 
methods  of  tunneling  through  soft  ground.  Some  of  these, 
like  the  quicksand  method  and  the  shield  method,  differ  in  char- 
acter entirely,  while  in  others,  like  the  Belgian,  German,  Eng- 
lish, Austrian,  and  Italian  methods,  the  difference  consists 
simply  in  the  different  order  in  which  the  drifts  and  headings 
are  driven,  in  the  difference  in  the  number  and  size  of  these 
advance  galleries,  and  in  the  different  forms  of  strutting  frame- 
work employed.  In  this  book  the  shield  method  is  considered 
individually ;  but  the  description  of  the  Belgian,  German,  Eng- 
lish, Austrian,  Italian,  and  quicksand  methods  are  grouped 
together  in  this  and  the  three  succeeding  chapters  to  permit  of 
easy  comparison. 

THE    BELGIAN  METHOD    OF   TUNNELING   THROUGH  SOFT 

GROUND. 

The  Belgian  method  of  tunneling  through  soft  ground  was 
first  employed  in  1828  in  excavating  the  Charleroy  tunnel  of 
the  Brussels-Charleroy  Canal  in  Belgium,  and  it  takes  its  name 
from  the  country  in  which  it  originated.  The  distinctive  char- 
acteristic of  the  method  is  the  construction  of  the  roof  arch 


TUNNELS  THROUGH  SOFT  GROUND 


135 


before  the  side  walls  and  invert  are  built.  The  excavation, 
therefore,  begins  with  the  driving  of  a  top  center  heading 
which  is  enlarged  until  the  whole  of  the  section  above  the 
springing  lines  of  the  arch  is  opened.  Various  modifications 
of  the  method  have  been  developed,  and  some  of  the  more 
important  of  these  will  be  described  farther  on,  but  we  shall 
begin  its  consideration  here  by  describing  first  the  original  and 
usual  mode  of  procedure. 

Excavation.  —  Fig.  61  is  the  excavation  diagram  of  the  Bel- 
gian method  of  tunneling.  The  excavation  is  begun  by  open- 
ing the  center  top  heading  No.  1,  which  is  carried  ahead  a 
greater  or  less  distance,  depending  upon  the  nature  of  the  soil, 
and  is  immediately  strutted.  This  heading  is  then  deepened 


FIGS.  61  and  62.  — Diagrams  Showing  Sequence  of  Excavations  in  the  Belgian  Method. 

by  excavating  part  No.  2,  to  a  depth  corresponding  to  the 
springing  lines  of  the  roof  arch.  The  next  step  is  to  remove 
the  two  side  sections  No.  3,  by  attacking  them  at  the  two  fronts 
and  at  the  sides  with  four  gangs  of  excavators.  The  regularity 
and  efficiency  of  the  mode  of  procedure  described  consist  in 
adopting  such  dimensions  for  these  several  parts  of  the  section 
that  each  will  be  excavated  at  the  same  rate  of  speed.  When 
the  upper  part  of  the  section  has  been  excavated  as  described, 
the  roof  arch  is  built,  with  its  feet  supported  by  the  unexca- 
vated  earth  below.  This  portion  of  the  section  is  excavated  by 
taking  out  first  the  central  trench  No.  4  to  the  depth  of  the 
bottom  of  the  tunnel,  and  then  by  removing  the  two  side  parts 
No.  5.  As  these  side  parts  No.  5  have  to  support  the  arch, 


136  TUNNELING 

they  have  to  be  excavated  in  such  a  way  as  not  to  endanger  it. 
At  intervals  along  the  central  trench  No.  4,  transverse  or  side 
trenches  about  2  ft.  wide  are  excavated  on  both  sides,  and 
struts  are  inserted  to  support  the  masonry  previously  supported 
by  the  earth  which  has  been  removed.  The  next  step  is  to 
widen  these  side  trenches,  and  insert  struts  until  all  of  the 
material  in  parts  No.  5  is  taken  out. 

When  the  material  penetrated  is  firm  enough  to  permit,  the 
plan  of  excavation  illustrated  by  the  diagram,  Fig.  62,  is  substi- 
tuted for  the  more  typical  one  just  described.  The  only  differ- 
ence in  the  two  methods  consists  in  the  plan  of  excavating  the 
upper  part  of  the  profile,  which  in  the  second  method  consists 
in  driving  first  the  center  top  heading  No.  1,  and  then  in  tak- 
ing out  the  remainder  of  the  section  above  the  springing  lines 
of  the  arch  in  one  operation,  while  in  the  first  method  it  is  done 
in  two  operations.  The  distance  ahead  of  the  masonry  to 
which  the  various  parts  can  be  driven  varies  from  10  ft.  to,  in 
some  cases,  100  ft.,  being  very  short  in  treacherous  ground,  and 
longer  the  more  stable  the  material  is. 

Strutting The  longitudinal  method  of  strutting,  with  the 

poling-boards  running  transversely  of  the  tunnel,  is  always 
employed  in  the  Belgian  method  of  tunneling.  In  driving  the 
first  center  top  heading,  pairs  of  vertical  posts  carrying  a  trans- 
verse cap-piece  are  erected  at  intervals.  On  these  cap-pieces 
are  carried  two  longitudinal  bars,  which  in  turn  support  the 
saddle  planks.  As  fast  as  part  No.  2,  Fig.  61,  is  excavated, 
the  vertical  posts  are  replaced  by  the  batter  posts  A  and  j5, 
Fig.  63.  The  excavation  of  parts  No.  3  is  begun  at  the  top, 
the  poling-boards  a  and  b  being  inserted  as  the  work  pro- 
gresses. To  support  the  outer  ends  of  these  poling-boards,  the 
longitudinals  X  and  Y  are  inserted  and  supported  by  the  batter 
posts  C  and  D.  In  exactly  the  same  way  the  poling-boards  c 
and  d,  the  longitudinals  Fand  W,  and  the  struts  E  and  F,  are 
placed  in  position ;  and  this  procedure  is  repeated  until  the 
whole  top  part  of  the  section  is  strutted,  as  shown  by  Fig.  63, 


TUNNELS  THROUGH  SOFT  GROUND 


137 


the  cross  struts  a:,  #,  z,  etc.,  being  inserted  to  hold  the  radial 
struts  firmly  in  position.  The  feet  of  the  various  radial 
props  rest  on  the  sill  M  N.  These  fan-like  timber  structures 
are  set  up  at  intervals  of  from  3  ft.  to  6  ft.,  depending  *npon 
the  quality  of  the  soil  penetrated. 


FIG.  63.—  Sketch  Showing  Radial  Roof  Strutting,  Belgian  Method. 

Centers.  —  Either  plank  or  trussed  centers  may  be  employed 
in  laying  the  roof  arch  in  the  Belgian  method,  but  the  form  of 
center  commonly  employed  is  a  trussed  center  constructed  as 
shown  by  Fig.  64.  It  may  be  said  to  consist  of  a  king-post 
truss  carried  on  top  of  a  modified  form  of  queen-post  truss. 
The  collar-beam  and  the  tie-beam  of  the  queen-post  truss  are 
spaced  about  7  ft.  apart,  and 
the  posts  themselves  are  left  far 
enough  apart  to  allow  the  pas- 
sage of  workmen  and  cars  be- 
tween them.  The  tie  beam  of 
the  king-post  truss  is  clamped 
to  the  collar-beam  of  the  queen- 
post  truss  by  iron  bands.  On 
the  rafters  of  the  two  trusses  are  fastened  timbers,  with  their 
outer  edges  cut  to  the  curve  of  the  roof  arch.  These  centers 
.are  set  up  midway  between  the  fan-like  strutting  frames  previ- 
ously described.  They  are  usually  built  of  square  timbers. 
The  tie  beams  are  usually  6x6  in.,  and  the  struts  and  posts 
4  x  4  in.  timbers.  The  reason  for  giving  the  larger  sectional 


FIG.    64.  — Sketch    Showing   Roof    Arch 
Center,  Belgian  Method. 


138  TUNNELING 

dimensions  to  the  tie  beams,  contrary  to  the  usual  practice  in 
constructing  centers,  is  that  it  has  to  serve  as  a  sill  for  distrib- 
uting the  pressure  to  the  foundation  of  unexcavated  soil 
which,  supports  the  center.  Sometimes  a  sub-sill  is  used  to 
support  the  -  center  upon  the  soil ;  and  in  any  case  wedges  are 
employed  to  carry  it,  which  can  be  removed  for  the  purpose  of 
striking  the  center.  After  the  arch  is  completed,  the  centers 
may  be  removed  immediately,  or  may  be  left  in  position  until 
the  masonry  has  thoroughly  set.  In  either  case  the  leading 
center  over  which  the  arch  masonry  terminates  temporarily  is 
left  in  position  until  the  next  section  of  the  arch  is  built. 

Masonry.  —  The  masonry  of  the  roof  arch,  which  is  the  first 
part  built,  is  of  necessity  begun  at  the  springing  lines,  and  the 
first  course  rests  on  short  lengths  of  heavy  planks.  These 
planks,  besides  giving  an  even  surface  upon  which  to  begin  the 
masonry,  are  essential  in  furnishing  a  bearing  to  the  struts 
inserted  to  support  the  arch  while  the  earth  beloAV  them,  part 
No.  5,  Fig.  61,  is  being  excavated.  As  the  arch  masonry 
progresses  from  the  springing  lines  upward,  the  radial  posts 
of  the  strutting  are  removed,  and  replaced  by  short  struts  rest- 
ing on  the  lagging  of  the  centers,  which  support  the  crown 
bars  or  longitudinals  until  the  masonry  is  in  place,  when  they 
and  the  poling-boards  are  removed,  and  the  space  between  the 
arch  masonry  and  walls  of  the  excavation  is  filled  with  stone 
or  well-rammed  earth. 

Considering  now  the  side  wall  masonry,  it  will  be  re- 
membered that  in  excavating  the  part  No.  5,  Fig.  61,  of  the 
section,  frequent  side  trenches  were  excavated,  and  struts 
inserted  to  take  the  weight  of  the  masonry.  These  struts  are 
inserted  on  a  batter,  with  their  feet  near  the  center  of  the 
tunnel  floor,  so  that  the  side  wall  masonry  may  be  carried  up 
behind  them  to  a  height  as  near  as  possible  to  the  springing 
lines  of  the  arch.  When  this  is  done  the  struts  are  removed, 
and  the  space  remaining  between  the  top  of  the  partly  fin- 
ished side  wall  and  the  arch  is  filled  in.  This  leaves  the  arch 


TUNNELS  THROUGH  SOFT  GROUND 


139 


supported  by  alternate  lengths  or  pillars  of  unexcavated  earth 
and   completed   side    wall.     The  next   step    is  to  remove  the 
remaining  sections  of  earth  between  the  sections  of  side  wall, 
and  fill  in  the  space  with  masonry. 
Fig.  65  is  a  cross-section,  showing 
the  masonry  completed  for  one-half 
and  the  inclined  props  in  position 
for  the  other  half;  and  Fig.  ftfc  is 
a  longitudinal  section  showing  the 
pillars    of    unexcavated    earth   be- 
tween the  consecutive  sets   of   in- 
clined   struts    and    several     other 
details  of  the  lining,  strutting,  and 
excavating  work. 

The  invert  masonry  is  built  after 
the  side  walls  are  completed.  This 
is  regarded  as  a  defect  of  tins  method  of  tunneling,  since  the 
lateral  pressures  may  squeeze  the  side  walls  together  and  dis- 
tort the  arch  l>efore  the  invert  is  in  place  to  brace  them  apart. 


FIG.  65.  —  Sketch  Showing  Method  of 
Underpining  Roof  Arch  with  the 
Side  Wall  Masonry. 


To  prevent  as  much  as  possible 
the  distortion  of  the  arch  after 
the  centers  are  removed,  it  is 
considered  good  practice  to 

shore     the     maSOUry    with    hoii- 
_  .,      .  , 

zontal  beams  having  their  ends 
abutting  against  plank,  as  shown  by  Fig.  65.  These  hori- 
zontal beams  should  be  placed  at  close  intervals,  and  be 
supported  at  intermediate  points  by  vertical  posts,  as  shown 


FIG.  66.—  Longitudinal  Section  Showing 
Construction  by  the  Belgian  Method. 


140  TUNNELING 

by  the  illustration.  Since  the  roof  arch  rests  for  some  time 
.supported  directly  by  the  unexcavated  earth  below,  settle- 
ment is  liable,  particularly  in  working  through  soft  ground. 
This  fact  may  not  be  very  important  so  long  as  the  settle- 
ment is  uniform,  and  is  not  enough  to  encroach  011  the  space 
necessary  for  the  safe  passage  of  travel.  To  prevent  the 
latter  possibility  the  centers  are  placed  from  9  ins.  to  15  ins. 
higher  than  their  true  positions,  depending  upon  the  nature  of 
the  soil,  so  that  considerable  settlement  is  possible  without  any 
•danger  of  the  necessary  cross-section  being  infringed  upon. 
In  conclusion  it  may  be  noted  that  the  lining  may  be  con- 
structed in  a  series  of  consecutive  rings,  or  as  a  single  cylin- 
drical mass. 

Hauling.  —  Since  in  this  method  of  tunneling  the  upper  part 
of  the  section  is  excavated  and  lined  before  the  excavation  of 
the  lower  part  is  begun,  the  upper  portion  is  always  more  ad- 
vanced than  the  lower.  To  carry  away  the  earth  excavated  at 
the  front,  therefore,  an  elevation  has  to  be  surmounted ;  and 
this  is  usually  done  by  constructing  an  inclined  plane  rising 
from  the  floor  of  the  tunnel  to  the  floor  of  the  heading,  as  shown 
by  Fig.  66.  This  inclined  plane  has,  of  course,  to  be  moved  ahead 
as  the  work  advances,  and  to  permit  of  this  movement  with  as 
little  interruption  of  the  other  work  as  possible,  two  planes  are 
employed.  One  is  erected  at  the  right-hand  side  of  the  section, 
and  serves  to  carry  the  traffic  while  the  left-hand  side  of  the 
lower  section  is  being  removed  some  distance  ahead  and  the 
other  plane  is  being  erected.  The  inclination  given  to  these 
planes  depends  upon  the  size  of  the  loads  to  be  hauled,  but  they 
.should  always  have  as  slight  a  grade  as  practicable.  Narrow- 
gauge  tracks  are  laid  on  these  planes  and  along  the  floor  of  the 
upper  part  of  the  section  passing  through  the  center  opening 
mentioned  before  as  being  left  in  the  centers  and  strutting. 

In  excavating  the  top  center  heading  there  is,  of  course,  an- 
other rise  to  its  floor  from  the  floor  of  the  upper  part  of  the 
.section.  Where,  as  is  usually  the  case  in  soft  soils,  this  top 


TUNNELS    THROUGH   SOFT   GROUND  141 

heading  is  not  driven  very  far  in  advance,  the  earth  from  the 
front  is  usually  conveyed  to  the  rear  in  wheelbarrows,  and 
dumped  into  the  cars  standing  on  the  tracks  below.  In%nnn 
soils,  where  the  heading  is  driven  too  far  in  advance  to  make 
this  method  of  conveyance  inadequate,  tracks  are  also  laid  on 
the  floor  of  the  heading,  and  an  inclined  plane  is  built  connect- 
ing it  with  the  tracks  on  the  next  level  below.  In  place  of 
these  inclined  planes,  and  also  in  place  of  those  between  the  floor 
of  the  tunnel  and  the  level  above,  some  form  of  hoisting  device 
is  sometimes  employed  to  lift  the  cars  from  one  level  to  the 
other.  There  are  some  advantages  to  this  method  in  point  of 
economy,  but  the  hoisting-machines  are  not  easily  worked  in. 
the  darkness,  and  accidents  are  likely  to  occur. 

In  the  advanced  top  heading  and  in  the  upper  part  of  the 
section  narrow-gauge  tracks  are  necessarily  employed,  and  these 
may  be  continued  along  the  floor  of  the  finished  section,  or  the 
permanent  broad-gauge  railway  tracks  may  be  laid  as  fast  as 
the  full  section  is  completed.  In  the  former  case  the  perma- 
nent tracks  are  not  laid  until  the  entire  tunnel  is  practically 
completed ;  and  in  the  latter  case,  unless  a  third  rail  is  laid,  the 
loads  have  to  be  transshipped  from  the  broad-  to  the  narrow- 
gauge  tracks  or  vice  versa.  It  is  the  more  general  practice  to 
use  a  third  rail  rather  than  to  transship  every  load. 

Modifications.  —  Considering  the  extent  to  which  the  Belgian 
method  of  tunneling  has  been  employed,  it  is  not  surprising 
that  many  modifications  of  the  standard  mode  of  procedure 
have  been  developed.  The  modification  which  differs  most 
from  the  standard  form  is,  perhaps,  that  adopted  in  excavating 
the  Roosebeck  tunnel  in  Germany.  This  method  preserves  the 
principal  characteristic  of  the  Belgian  method,  which  is  the 
construction  of  the  upper  part  of  the  section  first ;  but  instead 
of  building  the  side  walls  from  the  bottom  upward,  they  are 
built  in  small  sections  from  the  top  downward.  The  excavation 
begins  by  driving  the  center  top  heading  No.  1,  Fig.  67,  whose 
floor  is  at  the  level  of  the  springing  lines  of  the  roof  arch,  and 


14  -2 


TUNNELING 


then  the  two  side  parts  No.  2  are  excavated,  opening  up  the 
entire  upper  portion  of  the  section  in  which  the  roof  arch  is 
built,  as  in  the  regular  Belgian  method.  The  next  step  is  to 
excavate  part  No.  3,  shoring  up  the  arch 
at  frequent  intervals.  Between  these  sets 
of  shoring  the  side  walls  are  built,  resting 
planks  on  the  floor  of  part  No.  3,  and.  then 
the  sets  of  shores  are  removed  and  re- 
placed by  masonry.  Next  part  No.  4  is 
excavated,  shored,  and  filled  with  masonry 
as  was  part  No.  3.  In  exactly  the  same 


FIG.    67.  —  Diagram  Show- 
ing Sequence  of  Excava-    way  parts  5,  6,  7,  and  8  are   constructed 

tion  in  Modified  Belgian 
Method. 


in  the  order  numbered.     To  prevent  the 
distortion  of  the  arch  during  the  side-wall 
is    braced    by  horizontal  struts,    as  described 


•construction    it 
above  in  Fig.  65. 

Advantages,  —  The  advantages  of  the  Belgian  method  of 
tunneling  may  be  summarized  as  follows:  (1)  The  excavation 
progresses  simultaneously  at  several  points  without  the  differ- 
ent gangs  of  excavators  interfering  with  each  other,  thus  secur- 
ing rapidity  and  efficiency  of  work;  (2)  the  excavation  is  done 
by  driving  a  number  of  drifts  or  parts  of  small  section,  which 
are  immediately  strutted,  thus  causing  the  minimum  disturb- 
ance of  the  surrounding  material ;  (3)  the  roof  of  the  tunnel, 
which  is  the  part  of  the  lining  exposed  to  the  greatest  pressures, 
is  built  first. 

Disadvantages. : —  The  disadvantages  of  the  Belgian  method 
of  tunneling  may  be  summarized  as  follows :  (1)  The  roof  arch 
which  rests  at  first  on  compressible  soil  is  liable  to  sink ;  (2) 
before  the  invert  is  built  there  is  danger  of  the  arch  and  side 
walls  being  distorted  or  sliding  under  the  lateral  pressures;  (3) 
the  masonry  of  the  side  walls  has  to  be  underpinned  to  the  arch 
masonry. 

Accidents  and  Repairs. —  One  of  the  most  frequent  accidents 
in  the  Belgian  method  of  tunneling  is  the  sinking  of  the  roof 


TUNNELS  THROUGH  SOFT  GROUND 


arch  owing  to  its  unstable  foundation  on  the  unexcavated  soil 
of  the  lower  portion  of  the  section.  The  amount  of  settlement 
may  vary  from  a  few  inches  in  firm  soil  to  over  2  ft.  in  Joose 
soils.  To  counteract  the  effect  of  this  settlement  it  is  the  gene- 
ral practice  to  build  the  arch  some  inches  higher  than  its  nor- 
mal position.  When  the  settlement  is  great  enough  to  infringe 
seriously  upon  the  tunnel  section,  repairs  have  to  be  made ;  and 
the  only  way  of  accomplishing  them  is  to  demolish  the  arch  and 
rebuild  it  from  the  side  walls.  It  is  usually  considered  best  not 
to  demolish  the  arch  until  the  invert  has  been  placed,  so  that 

no   further  disturbance   is   likely  once 
the  lining  is  completed  anew. 

The  rotation  of  the  arch  about  its 
keystone,  or  the  opening  of  the  arch  at 
the  crown,  by  the  squeezing  inward  of 
the  haunches  by  the  lateral  pressures, 
is  another  characteristic  accident.  Fig. 
68  shows  the  nature  of  the  distortion 
produced ;  the  segments  of  the  arch 
move  toward  each  other  by  revolving 
on  the  intradosal  edges  of  the  keystone, 
which  are  broken  away  and  crushed  together  with  the  operation, 
while  the  extradosal  edges  are  opened.  It  is  to  prevent  this 
occurrence  that  the  horizontal  struts  shown  in  Fig.  65  are  em- 
ployed. The  manner  of  repairing  this  accident  differs,  depend- 
ing upon  the  extent  of  the  injury7.  When  the  intradosal  edges 
of  the  keystone  are  but  slightly  crushed,  the  repairing  is  done 
as  directed  by  Fig.  69.  When  the  keystone  is  completely 
crushed,  however,  the  indications  are  that  the  material  of  the 
keystone,  usually  brick,  is  not  strong  enough  to  resist  the 
pressures  coming  upon  it,  and  it  is  advisable  to  substitute  a 
stronger  material  in  the  repairs,  and  a  stone  keystone  is  con- 
structed as  shown  by  Fig.  TO.  The  middle  stone  of  this  key- 
stone extends  through  the  depth  of  the  arch  ring,  and  the  two 
side  stones  only  half-way  through,  their  purpose  being  merely 


FIG.  68.  —  Sketch  Showing 
Failure  of  Roof  Arch  by 
Opening  at  Crown. 


144 


TUNNELuSG 


to  resist  the  crushing  forces  which  are  greatest  at  the  intrados* 
Sometimes,  when  the  pressures  are  unsymmetrical,  the  arch 
ring  breaks  at  the  haunches  as  well  as  the  crown,  as  shown  by 


FIGS.  69  to  71.  —Sketches  Showing  Methods  of  Repairing  Roof  Arch  Failures. 

Fig.  71,  which  also  indicates  the  mode  of  repairing.  This 
consists  in  demolishing  the  original  arch,  and  rebuilding  it 
with  stone  voussoirs  inserted  in  place  of  the  brick  in  which  the 
rupture  occurred. 


GERMAN    METHOD 


145 


CHAPTER   XIV.  » 

THE    GERMAN    METHOD    OF    EXCAVATING 

TUNNELS    THROUGH    SOFT    GROUND; 

BALTIMORE    BELT    LINE    TUNNEL. 


THE  German  method  of  tunneling  was  first  used  in  1803 
in  constructing  the  St.  Quentin  Canal.  In  1837  the  Konigs- 
dorf  tunnel  of  the  Cologne  and  Aix  la  Chapelle  R.R.  was 
excavated  by  the  same  method.  The  success  of  the  method  in 
these  two  difficult  pieces  of  soft-ground  tunneling  led  to  its 
extensive  adoption  throughout  Germany,  and  for  this  reason 
it  gradually  came  to  be  designated  as  the  German  method. 
Briefly  explained  the  method  consists  in  excavating  first  an 
annular  gallery  in  which  the  side  walls  and  roof  arch  are  built 
complete  before  taking  out  the  center  core  and  building  the 
invert. 

Excavation. — The  excavation  of  tunnels  by  the  German 
method  is  begun  either  by  driving  two  bottom  side  drifts  or 
by  driving  a  center  top  heading.  Fig.  73  shows  the  mode  of 


FIGS.  72  and  73.  —  Diagrams  Showing  Sequence  of  Excavation  in  German  Method 
of  Tunneling. 

procedure  when  bottom  side  drifts  are  used  to  start  the  work. 
The  two  side  drifts  Xo.  1  are  made  from  7  ft.  to  8  ft.  wide, 
and  about  one-third  the  total  height  of  the  full  section ;  the 


146 


TUNNELING 


width  of  each  heading  has  to  be  sufficient  for  the  construction 
of  the  masonry  and  strutting,  and  for  the  passage  of  narrow 
spoil  cars  alongside  them.  These  drifts  are  increased  in  height 
to  the  springing  line  of  the  arch  by  taking  out  the  two  drifts 
No.  2.  Next  the  top  center  heading  No.  3  is  driven,  and 
finally  the  two  haunch  headings  No.  4  are  excavated.  The 
center  core  No.  5  is  utilized  to  support  the  strutting  until 
the  side  walls  and  roof  arch  are  completed,  when  it  is  broken 
down  and  removed.  In  case  of  very  loose  material,  where  the 
first  side  drifts  cannot  be  carried  as  high  as  one-third  the 
height  of  the  section,  it  is  the  common  practice  to  make  them 
about  one-fourth  the  height,  and  to  take  out  the  side  portions 
of  the  annular  gallery  in  three  parts,  as 
shown  by  Fig.  73. 

The  top  center  heading  plan  of  com- 
mencing the  excavation  is  usually  em- 
ployed in  firm  materials  or  when  a  vein 
of  water  is  encountered  in  the  upper  part 
of  the  section.  In  the  latter  contingency 
a  small  bottom  drift  A.,  Fig.  74,  is  first 


FIG.  74. -Diagram  show-     (Jriven  to  serve   as  a  drain;    but  in  any 

ing  Sequence  of  Excava- 
tions in  water  Bearing     case  the  excavation  proper  of  the  tunnel 

Material,     German  •    ,        .         ~  i    •     •  ,  i 

Method.  consists    in  first   driving   the    center    top 

heading  No.  1,  and  then  by  working  both 

ways  along  the  profile  parts,  Nos.  2,  3,  4,  and  5  are  removed. 

Part  No.  6  is  left  to  support  the  strutting  until  the  side  walls 

and  roof  arch  are  built,  when  it  is  also  excavated. 

Strutting.  —  -  When  the  excavation  is  begun  by  bottom  side 
drifts  these  drifts  are 'strutted  by  erecting  vertical  posts  close 
against  the  sides  of  the  drift  and  placing  a  cap-piece  trans- 
versely across  the  roof  of  the  drift.  The  side  posts  are 
usually  supported  by  sills  placed  across  the  bottom  of  the  drift. 
These  frameworks  of  posts,  cap,  and  sill  are  erected  at  short 
intervals,  and  the  roof,  and,  if  necessary,  the  sides  of  the  drift 
between  them,  are  sustained  by  means  of  longitudinal  poling- 


GERMAN    METHOD 


147 


boards  extending  from  one  frame  to  the  next.  The  cap-pieces 
of  the  strutting  for  the  bottom  drifts  serve  as  sills  for  the 
exactly  similar  strutting  of  the  heading  next  above.  To  sup- 
port the  additional  weight,  and  to  allow  the  construction  0£  the 
side  walls,  the  strutting  of  the  bottom  drifts  is  strengthened  by 
inserting  an  intermediate  post  between  the  original  side  posts 
of  each  frame.  These  intermediate  posts  are  not  inserted  at 
the  center  of  the  frames  or  bents,  but  close  to  the  wall  masonry 
line  as  shown  by  Fig.  75.  This  eccentric  position  of  the  post 


FIG.  75.  — Sketch  Showing  Work  of  Ex- 
cavating and  Timbering  Drifts  and 
Headings. 


FIG.    76.  — Sketch   Showing  Method  of 
Roof  Strutting. 


avoids  an}r  interference  with  the  hauling,  and  also  allows  the 
removal  of  the  adjacent  side  post  when  the  masonry  is 
constructed. 

Two  methods  of  strutting  the  soffit  of  the  excavation  are 
employed,  one  being  a  modification  of  the  longitudinal  system 
employed  in  the  English  method  of  tunneling  described  in  a 
succeeding  chapter,  and  the  other  a  modification  of  the  Belgian 
system  previously  described.  Fig.  76  shows  the  method  of 
«m ploying  the  radial  strutting  of  the  Belgian  system.  At  the 
beginning  the  center  top  heading  is  strutted  with  rectangular 
bents  such  as  are  employed  for  strutting  the  drifts.  As  this 
heading  is  enlarged  by  taking  out  the  haunch  sections,  radial 
posts  are  inserted,  as  shown  by  Fig.  76,  which  also  indicates 


148 


TUNNELING 


the  method  of  strutting  the  side  trenches  when  the  excavation 
is  carried  downward  from  the  center  top  heading  instead  of 
upward  from  bottom  side  drifts. 

Masonry.  —  Whatever  plan  of  excavation  or  strutting  is 
employed,  the  construction  of  the  masonry  lining  in  the  German 
method  of  tunneling  begins  at  the  foundations  of  the  side  walls 
and  is  carried  upward  to  the  roof  arch.  The  invert,  if  one  is 
required,  is  built  after  the  center  core  of  earth  is  removed. 

Centering.  -  -  Tunnel  centers  are  generally  employed  in  the 
German  method  of  tunneling,  a  common  construction  being 

shown  by  Fig.  77.  It  is  essen- 
tially a  queen-post  truss,  the  tie 
beam  of  which  rests  on  a  transverse 
sill  as  shown  by  the  illustration. 
The  transverse  sill  is  supported 
along  its  central  portion  by  the 
unexcavated  center  core  of  earth, 
and  at  its  ends  either  directly  on 
the  vertical  posts  or  on  longitudi- 
nal beams  resting  on  these  posts. 
The  diagonal  members  of  the 
queen-post  truss  form  the  bottom 
chords  of  small  king-post  trusses 

which  are  employed  to  build  out  the  exterior  member  of  the 
center  to  a  closer  approximation  to  the  curve  of  the  arch. 

Hauling.  —  When  the  bottom  side  drift  plan  of  excavation 
is  employed,  the  spoil  from  the  front  of  the  drift  is  removed  in 
narrow-gauge  cars  running  on  a  track  laid  as  close  as  practicable 
to  the  center  core.  These  same  cars  are  also  employed  to  take 
the  spoil  from  the  drifts  above,  through  holes  left  in  the  ceiling 
strutting  of  the  bottom  drifts.  The  spoil  from  the  soffit  sec- 
tions may  be  removed  by  the  same  car  lines  used  in  excavating 
the  drifts,  or  a  narrow-gauge  track  may  be  laid  on  the  top  of  the 
center  core  for  this  special  purpose.  In  the  latter  case  the  soffit 
tracks  are  usually  connected  by  means  of  inclined  planes  with 


PlG.    77.  —  Sketch  Showing  Roof  Arch 
Centers  and  Arch  Construction. 


GERMAN   METHOD  149 

the  tracks  on  the  bottoms  of  the  side  drifts.  Generally,  how- 
ever, the  separate  soffit  car  line  is  not  used  unless  the  material 
is  of  such  a  firm  character  that  the  headings  and  drifts  can  be 
carried  a  great  distance  ahead  of  the  masonry  work.  With  the 
center  top  heading  plan  of  beginning  the  excavation,  the  car 
track  has,  of  course,  to  be  laid  on  the  top  of  the  center  core. 
The  center  core  itself  is  removed  by  means  of  car  tracks  along 
the  floor  of  the  completed  tunnel. 

Advantages  and  Disadvantages.  —  Like  the  Belgian  method 
of  tunneling,  the  German  method  has  its  advantages  and  dis- 
advantages. Since  the  excavation  consists  at  first  of  a  narrow 
annular  gallery  only,  the  equilibrium  of  the  earth  is  not  greatly 
disturbed,  and  the  strutting  does  not  need  to  be  so  heavy  as  in 
methods  where  the  opening  is  much  larger.  The  undisturbed 
center  core  also  furnishes  an  excellent  support  for  the  strutting, 
and  for  the  centers  upon  which  the  roof  arches  are  built. 
Another  important  advantage  of  the  method  is  that  the  con- 
struction of  the  masonry  lining  is  begun  logically  at  the  bottom, 
and  progresses  upward,  and  a  more  homogeneous  and  stable 
construction  is  possible.  The  great  disadvantage  of  the  method 
is  the  small  space  in  which  the  hauling  has  to  be  done.  The 
spoil  cars  practically  fill  the  narrow  drifts  in  passing  to  and  from 
the  front,  and  interfere  greatly  with  the  work  of  the  carpenters 
and  masons.  Another  objection  to  the  method  is  that  the 
invert  is  the  very  last  portion  of  the  lining  to  be  built.  This 
may  not  be  a  serious  objection  in  reasonably  compact  and  stable 
materials,  but  in  very  loose  soils  there  is  always  the  danger  of 
the  side  Avails  being  squeezed  together  before  the  invert  masonry 
is  in  position  to  hold  them  apart.  Altogether  the  difficulties 
are  of  a  character  which  tend  to  increase  the  expense  of  the 
method,  and  this  is  the  reason  why  to-day  it  is  seldom  used 
even  in  the  country  where  it  was  first  developed,  and  for  some 
time  extensively  employed.  For  repairing  accidents,  such  as 
the  caving  in  of  completed  tunnels,  the  German  method  of  tun- 
neling is  frequently  used,  because  of  the  ease  with  which  the 


150  TUNNELING 

timbering   is  accomplished.      In  such  cases  the   cost   of    the 

method  used  cuts  a  small  figure,  so  long  as   it  is   safe  and 
expeditious. 


BALTIMORE    BELT    LINE    TUNNEL. 

The  Baltimore  Belt  Ry.  Co.  was  organized  in  1890  by 
officials  of  the  Baltimore  &  Ohio,  and  Western  Maryland  rail- 
ways, and  Baltimore  Capitalists,  to  build  7  miles  of  double  track 
railway,  mostly  within  the  city  limits  of  Baltimore.  This  rail- 
way was  partly  open  cut  and  embankment,  and  partly  tunnel,, 
and  its  object  was  to  afford  the  companies  named  facilities  for 
reaching  the  center  of  the  city  with  their  passengers  and  freight. 
To  carry  out  the  work  the  Maryland  Construction  Co.  was 
organized  by  the  parties  interested,  and  in  September,  1890,  this 
company  let  the  contract  for  construction  to  Rayan  &  McDon- 
ald of  Baltimore,  Md.  The  chief  difficulties  of  the  work  cen- 
tered in  the  construction  of  the  Howard-street  tunnel,  8,350  ft. 
long,  running  underneath  the  principal  business  section  of 
the  city. 

Material  Penetrated.  —  The  soil  penetrated  by  the  tunnel 
was  of  almost  all  kinds  and  consistencies,  but  was  chiefly  sand 
of  varying  degrees  of  fineness  penetrated  by  seams  of  loam, 
clay,  and  gravel.  Some  of  the  clay  was  so  hard  and  tough  that 
it  could  not  be  removed  except  by  blasting.  Rock  was  also 
found  in  a  few  places.  For  the  most  part,  however,  the  work 
was  through  soft  ground,  furnishing  more  or  less  water,  which 
necessitated  unusual  precautions  to  avoid  the  settling  of  the 
street,  and  consequent  damage  to  the  buildings  along  the  line. 
A  large  quantity  of  water  was  encountered.  Generally  this 
water  could  be  removed  by  drainage  and  pumps,  and  the  earth 
be  prevented  from  washing  in  by  packing  the  space  between  the 
timbering  with  hay  or  other  materials.  At  points  where  the 
inflow  was  greatest,  and  the  earth  was  washed  in  despite 
the  hay  packing,  the  method  was  adopted  of  driving  6-in.  per- 


GERMAN   METHOD  151 

forated  pipes  into  the  sides  of  the  excavation,  and  forcing 
cement  grout  through  them  into  the  soil  to  solidify  it  These 
pipes  penetrated  the  ground  about  10  ft.,  and  the  method 
proved  very  efficient  in  preventing  the  inflow  of  water? 

Excavation.  —  The  excavation  was  carried  out  according  to 
the  German  method  of  tunneling.  Bottom  side  drifts  were 
first  driven,  and  then  heightened  to  the  springing  line  of  the 
roof  arch.  Next  a  center  top  heading  was  driven,  and  the 
haunch  sections  taken  out.  The  object  of  beginning  the  exca- 
vations by  bottom  side  drifts,  was  to  drain  the  soil  of  the  upper 
part  of  the  section.  The  center  core  was  removed  after  the 
side  walls  and  roof  arch  were  completed,  its  removal  being 
kept  from  50  ft.  to  75  ft.  to  the  rear  of  the  advanced  heading. 
The  dimensions  of  the  side  drifts  proper  were  about  8x8  ft., 
but  they  were  often  carried  down  much  below  the  floor  level 
to  secure  a  solid  foundation  bed  for  the  side  walls. 

Strntting.  —  The  side  drifts  were  strutted  by  means  of 
frames  composed  of  two  batter  posts  resting  on  boards,  and 
having  a  cap-piece  extending  transversely  across  the  roof 
of  the  drift.  These  frames  were  spaced  about  4  ft.  apart. 
The  excavation  was  advanced  in  the  usual  way  by  driving 
poling-boards  at  the  top  and  sides,  with  a  slight  outward  and 
upward  inclination,  so  that  the  next  frame  could  be  easily 
inserted  with  additional  space  enough  between  it  and  the 
sheeting  to  permit  the  next  set  of  poling-boards  to  be  inserted. 
These  poling-boards  were  driven  as  close  together  as  practicable 
so  as  to  prevent  as  much  as  possible  the  inflow  of  water  and 
earth. 

The  center  top  heading  was  strutted  in  the  same  manner  as 
were  the  side  drifts.  The  arrangement  of  the  strutting  em- 
ployed in  enlarging  the  center  top  heading  is  shown  clearly  by 
Fig.  78,  which  also  shows  the  manner  of  strutting  the  side 
drifts  and  face  of  the  excavation,  and  of  building  the  masonry. 

Centers Both  wood  and  iron  centers  were  employed  in 

building  the  roof  arch.  The  timber  centering  was  constructed 


152 


TUXXEL1XG 


of  square  timbers,  as  shown  by  Fig.  79.  This  construction  of 
the  iron  centers  is  shown  by  Fig.  80.  Each  of  the  iron  centers 
consisted  of  two  6  x  6  in.  angles  butted  together,  and  bent  into 
the  form  of  an  arch  rib.  Six  of  these  ribs  were  set  up  4  ft. 
apart.  They  were  made  of  two  half  ribs  butted  together  at  the 
crown,  and  were  held  erect  and  the  proper  distance  apart  by 


PIG.  78. —  Sketch  Showing  Method  of  Excavating  and  Strutting  Baltimore  Belt 
Line  Tunnel. 

spacing  rods.  The  rearmost  rib  was  held  fast  to  the  completed 
arch  masonry,  and  in  turn  supported  the  forward  ribs  while  the 
lagging  was  being  placed. 

Masonry.  —  The  side  walls  of  the  lining  were  built  first  in 
the  bottom  side  drifts,  as  shown  by  Fig.  78.  They  were  gen- 
erally placed  on  a  foundation  of  concrete,  from  1  ft.  to  2  ft. 
thick.  As  a  rule  the  side  walls  were  not  built  more  than  20 
ft.  in  advance  of  the  arch,  but  occasionally  this  distance  was 
increased  to  as  much  as  90  ft.  The  roof  arch  consisted  ordina- 


, 


GERMAN    METHOD 


-    153 


rily  of  five  rings  of  brick,  but  at  some  places  in  especially  un- 
stable soil  eight  rings  of  brick  were  emplo}*ed.  The  arch  was 
built  in  concentric  sections  about  18  ft.  in  length.  All  the 
timber  of  the  strutting  above  the  arch  and  outside  of  the  side 
walls  was  left  in  place,  and  the  voids  were  filled  with  rubble 
masonry  laid  in  cement  mortar.  It  required  about  125  mason 


FIG.  79.  —  Roof  Arch  Construction  with  Timber  Centers,  Baltimore  Belt  Line  Tunnel. 

hours  to  build  an  18-ft.  arch  section.     Figs.  79  and  80  show 
various  details  of  the  masonry  arch  work. 

Owing  to  the  very  unstable  character  of  the  soil,  consider- 
able difficulty  was  experienced  in  building  the  masonry  invert. 
The  process  adopted  was  as  follows;  Two  parallel  12  -i-  12  in. 
timbers  were  first  placed  transversely  across  the  tunnel,  abutting 
against  longitudinal  timbers  or  wedges  resting  against  the  side 
walls.  Short  sheet  piles  were  then  driven  into  the  tunnel 


154  TUNNELING 

bottom  outside  of  these  timbers,  forming  an  inclosure  similar 
to  a  cofferdam,  from  which  the  earth  could  be  excavated  with- 
out disturbing  the  surrounding  ground.  The  earth  being 
excavated,  a  layer  of  concrete  8  ins.  thick  was  placed,  and  the 
brick  masonry  invert  constructed  on  it.  In  less  stable  ground 
each  of  the  above  described  cofferdams  was  subdivided  by 
transverse  timbers  and  sheet  piling  into  three  smaller  coffer- 
dams. Here  the  masonry  of  the  middle  section  was  first  con- 
structed, and  then  the  side  sections  built.  Where  the  ground 


PIG.  80.— Roof  Arch  Construction  with  Iron  Centers,  Baltimore  Belt  Line  Tunnel. 

was  worst,  still  more  care  was  necessary,  and  the  bottom  had 
to  be  covered  with  a  sheeting  of  l¥-in.  plank  held  down  by 
struts  abutting  against  the  large  transverse  timbers.  The 
invert  masonry  was  constructed  on  this  sheeting.  Refuge 
niches  9  ft.  high,  3  ft.  wide,  and  15  ins.  deep  were  built  in  the 
side  walls. 

Accidents.  —  In  this  tunnel,  owing  to  the  quick  striking  of 
the  centers,  it  was  found  that  the  masonry  lining  flattened  at 
the  crown  and  bulged  at  the  sides.  This  was  attributed  to  the 
insufficient  time  allowed  for  the  mortar  to  set  in  the  rubble 


GERMAN    METHOD  155 

filling.  Earth  packing  was  tried,  but  gave  still  worse  results. 
Finally  dry  rubble  rilling  was  adopted,  with  satisfactory  results. 
There  was  necessarily  some  sinking  of  the  surface.  This,  re- 
sulted partly  from  the  necessity  of  changing  and  removing  of 
the  timbers,  and  from  the  compression  and  springing  of  the 
timbers  under  the  great  pressures.  The  crown  of  the  arch  also 
settled  from  2  ins.  to  6  ins.,  due  to  the  compression  of  the 
mortar  in  the  joints.  The  maximum  sinking  of  the  surface  of 
the  street  over  the  tunnel  was  about  18  ins.;  it  usually  ran 
from  1  to  12  ins.  Some  damage  was  done  to  the  water  and  gas 
mains.  This  damage  was  not  usually  serious,  but  it  of  course 
necessitated  immediate  repairs,  and  in  some  instances  it  was 
found  best  to  reconstruct  the  mains  for  some  distance.  At  one 
point  along  the  tunnel  where  very  treacherous  material  was 
found,  the  surface  settlement  caused  the  collapse  of  an  adjacent 
building,  and  necessitated  its  reconstruction. 


156  TUNNELING 


CHAPTER   XV. 

THE  FULL   SECTION   METHOD  CF  TUNNELING: 
ENGLISH    METHOD  ;    AUSTRIAN    METHOD. 


ENGLISH  METHOD. 

THE  English  method  of  tunneling  through  soft  ground,  as 
its  name  implies,  originated  in  England,  where,  owing  to  the 
general  prevalence  of  comparatively  firm  chalks,  clays,  shales, 
and  sandstones,  it  has  gained  unusual  popularity.  The  dis- 
tinctive characteristics  of  the  method  are  the  excavation  of  the 
full  section  of  the  tunnel  at  once,  the  use  of  longitudinal  strut- 
ting, and  the  alternate  execution  of  the  masonry  work  and 
excavation.  In  America  the  method  is  generally  designated  as 
the  longitudinal  bar  method,  owing  to  the  mode  of  strutting, 
which  has  gained  particular  favor  in  America,  and  is  commonly 
employed  there  even  when  the  mode  of  excavation  is  distinc- 
tively German  or  Belgian  in  other  respects. 

Excavation.  —  Although,  as  stated  above,  the  distinctive 
characteristic  of  the  English  method  is  the  excavation  of  the 
full  section  at  once,  the  digging  is  usually  started  by  driving 
a  small  heading  or  drift  to  locate  and  establish  the  axis  of  the 
tunnel,  and  to  facilitate  drainage  in  wet  ground.  These  ad- 
vance galleries  may  be  driven  either  in  the  upper  or  in  the 
lower  part  of  the  section,  as  the  local  conditions  and  choice 
of  the  engineer  dictate.  Whether  the  advance  gallery  is  located 
at  the  top  or  at  the  bottom  of  the  section  makes  no  difference  in 
the  mode  of  enlarging  the  profile.  This  work  always  begins 
at  the  upper  part  of  the  section.  A  center  top  heading  is 
driven  and  strutted  by  erecting  posts  carrying  longitudinal  bars 
supporting  transverse  poling-boards.  This  heading  is  imme- 


THE   FULL    SECTION   METHOD 


157 


tion   in  English  Method 
of  Tunneling. 


diately  widened  by  digging  away  the  earth  at  each  side,  and  by 
strutting  the  opening  by  temporary  posts  resting  on  blocking, 
and  carrying  longitudinal  bars  supporting  poling-boards.  This 
process  of  widening  is  continued  in  this  manner  until  thv  full 
roof  section,  No.  1,  Fig.  81,  is  opened,  when  a  heavy  transverse 
sill  is  laid,  and  permanent  struts  are 
erected  from  it  to  the  longitudinal  bars, 
the  temporary  posts  and  blocking  being 
removed.  The  excavation  of  part  No.  2 
then  begins  by  opening  a  center  trench 
and  widening  it  on  each  side,  temporary 
posts  being  erected  to  support  the  sill 
above.  As  soon  as  part  No.  2  is  fully  ex- 
cavated, a  second  transverse  sill  is  placed  Fm-  si. -Diagram  Show 

mg  Sequence  of  Excava- 

below   the   first,    and    struts    are  ,  placed 

between   them.     The  excavation  of    part 

No.  3  is  carried  out  in  exactly  the  same  manner  as  was  part 

No.  2.     The  lengths  of  the  various  sections,  Nos.  1,  2,  and  3, 

generally   run   from    12    ft.    to    20    ft.,    depending   upon    the 

character  of  the  soil. 

Strutting The  strutting  in  the  English  method  of  tunnel- 
ing consists  of  a  transverse  framework  set  close  to  the  face  of 
the  excavation,  which  supports  one  end  of  the  longitudinal 
crown  bars,  the  other  ends  of  which  rest  on  the  completed 
lining.  The  transverse  framework  is  composed  of  three  hori- 
zontal sills  arranged  and  supported  as  shown  by  Fig.  82.  The 
bottom  sill  A  is  carried  by  vertical  posts  resting  on  blocking  on 
the  floor  of  the  excavation.  From  the  bottom  sill  vertical 
struts  rise  to  support  the  middle  sill  B.  The  top  sill,  or  miners' 
sill  (7,  is  carried  by  vertical  posts  or  struts  rising  from  the 
middle  sill  B.  The  vertical  struts  are  usually  round  timbers 
from  6  ins.  to  8  ins.  in  diameter ;  and  the  sills  are  square  tim- 
bers of  sufficient  section  to  carry  the  vertical  loads,  and  gener- 
ally made  up  of  two  posts  scarf-jointed  and  butted  to  permit 
them  to  be  more  easily  handled.  In  firm  soils  the  struts  be- 


158 


TUNNELING 


tween  the  sills  are  all  set  vertically,  but  those  at  the  extreme 
sides  of  the  roof  section  are  inclined.  In  loose  soils,  however, 
where  the  sides  of  the  excavation  must  be  shored,  the  V- 
bracing  shown  by  Fig.  82  is  employed  between  one  or  more 
pairs  of  sills  as  the  conditions  necessitate.  The  manner  of 
holding  the  transverse  framework  upright  is  explained  quite 
clearly  by  Fig.  83  ;  inclined  props  extending  from  the  com- 
pleted masonry  to  the  sills  of  the  framework  being  employed. 
Two  props  are  used  to  each  sill.  Sometimes,  in  addition  to  the 


FIGS.  82  and  83.  —  Sketches  Showing  Construction  of  Strutting,  English  Method. 

props  shown,  another  nearly  horizontal  prop  extends  from  the 
crown  of  the  arch  masonry  to  the  middle  piece  of  the  strutting. 
Referring  to  Fig.  83,  it  will  be  observed  that  the  longitudinal 
crown  bars  are  above  the  extrados  of  the  roof  arch.  When, 
therefore,  the  lining  masonry  has  been  completed  close  up  to 
the  transverse  framework,  the  latter  is  removed,  leaving  the 
crown  bars  resting  on  the  arch  masonry ;  and  excavation,  which 
has  been  stopped  while  the  masonry  was  being  laid,  is  continued 
for  another  12  ft.  to  20  ft,  and  the  transverse  framework  is 
erected  at  the  face,  and  braced  or  propped  against  the  completed 
lining  as  shown  by  Fig.  83.  The  next  step  is  to  place  the 


THE    FULL    SECTION    METHOD 


159 


crown  bars,  and  this  is  done  by  pulling  them  ahead  from  their 
original  position  over  the  masonry  of  the  completed  section  of 
the  roof  arch.  It  will  be  understood  that  the  crown  bars  are 
not  pulled  ahead  their  full  length  at  one  operation,  bvt  are 
advanced  by  successive  short  movements  as  the  excavation 
progresses,  their  outer  ends  being  supported  by  temporary 
posts  until  the  transverse  framework  is  built  at  the  face  of  the 
excavation. 

Centers Two  standard  forms  of  centers  are  employed  in 

the  English  method  of  tunneling,  as  shown  by  Figs.  84  and  85. 
Both  consist  of  an  outer  portion,  constructed  much  like  a 
typical  plank  center,  which  is  strengthened  against  distortion 
by  an  interior  truss  framework.  The  elemental  members  of 


FIGS.  84  and  85.—  Sketches  of  Typical  Timber  Roof-Arch  Centers,  English  Method. 

this  truss  framework  take  the  form  of  a  queen-post  truss,  as  is 
shown  more  particularly  by  Fig.  84.  In  Fig.  85  the  queen- 
post  truss  construction  is  less  easily  distinguished,  owing  to 
the  cutting  of  the  bottom  tie-beam  and  other  modifications,  but 
it  can  still  be  observed.  The  possibility  of  cutting  the  tie-beam 
as  shown  in  Fig.  85,  without  danger,  is  due  to  the  fact  that 
the  lateral  pressures  on  the  haunches  of  the  center  counteract 
the  tendency  of  the  center  to  flatten  under  load,  which  is 
usually  counteracted  by  the  tie-beam  alone.  The  object  of 
cutting  the  tie-beam  is  to  afford  room  for  the  props  running 
from  the  completed  masonry  to  the  transverse  framework  of 
the  strutting  as  shown  by  Fig.  83. 

Generally  four  or  five  centers  are  used  for  each  length  of 
arch  built.       They  are   set  up  so  that  the   tie-beams  rest  on 


160  TUNNELING 

double  opposite  wedges  carried  by  a  transverse  beam  below. 
This  transverse  beam  in  turn  rests  on  another  transverse  beam 
which  is  supported  by  posts  carried  on  blocking  on  the  invert 
masonry.  It  is  usually  made  with  a  butted  joint  at  the  middle 
to  permit  its  removal,  since  it  is  so  long  that  the  masonry  has 
to  be  built  around  its  extreme  ends.  The  lagging  is  of  the 
usual  form,  and  rests  on  the  exterior  edges  of  the  curved  upper 
member  of  the  centers. 

Masonry.  —  In  the  English  method  of  tunneling,  the  masonry 
begins  with  the  construction  of  the  invert,  and  proceeds  to  the 
crown  of  the  arch.  The  lining  is  built  in  lengths,  or  successive 
rings,  corresponding  to  the  length  of  excavation,  which,  as  pre- 
viously stated,  is  from  12  ft.  to  20  ft.  Each  ring  or  length  of 
lining  terminates  close  to  the  transverse  strutting  frame  erected 
at  the  face  of  the  excavation.  Work  is  first  begun  on  the 
invert  at  the  point  where  the  preceding  ring  of  masonry  ends, 
and  is  continued  to  the  transverse  strutting  frame  at  the  front 
of  the  excavation.  As  fast  as  the  invert  is  completed,  work  is 
begun  on  the  side  walls.  In  very  loose  soils  the  longitudinal 
bars  supporting  the  sides  of  the  excavation  are  removed  after 
the  side  walls  are  built ;  but  in  firmer  soils  they  may  be  taken 
out  one  by  one  just  ahead  of  the  masonry,  or  in  very  firm  soils 
it  may  be  possible  to  remove  them  entirely  before  beginning 
the  side  walls.  In  all  cases  it  is  necessary  to  fill  the  space 
between  the  masonry  and  the  walls  of  the  excavation  with  rip- 
rap or  earth.  To  build  the  roof  arch  the  centers  are  first 
erected  as  described  above,  and  the  crown  bars  are  removed  as 
previously  described  by  putting  them  ahead  after  the  arch  ring 
is  completed.  As  with  the  side  walls,  the  vacant  space  be- 
tween the  arch  ring  and  the  roof  of  the  excavation  must 
be  filled  in.  Usually  earth  or  small  stones  are  used  for  filling ; 
but  in  very  loose  soils  it  is  sometimes  the  practice  not  to 
remove  the  poling-boards,  but  to  support  them  by  short  brick 
pillars  resting  on  the  arch  ring  and  then  to  fill  around  these 
pillars. 


THE   FULL   SECTION   METHOD  161 

Hauling.  —  To  haul  away  the  material  and  take  in  supplies, 
tracks  are  laid  on  the  invert  masonry.  Generally  the  perma- 
nent tracks  are  laid  as  fast  as  the  lining  is  completed.  A  short 
section  of  temporary  track  is  used  to  extend  this  permanent 
track  close  to  the  work. 

Advantages  and  Disadvantages.  —  The  great  advantage  of  the 
English  method  of  tunneling  is  that  the  masonry  lining  is 
built  in  one  piece  from  the  foundations  to  the  crown,  making 
possible  a  strong,  homogeneous  construction.  It  also  pos- 
sesses a  decided  advantage  because  of  the  simple  methods  of 
hauling  which  are  possible :  there  being  no  differences  of  level 
to  surmount,  no  hoisting  of  cars  nor  trans-shipments  of  loads 
are  necessary.  The  chief  disadvantage  of  the  method  is  that 
the  excavators  and  masons  work  alternately,  thus  making  the 
progress  of  the  work  slower  perhaps  than  in  any  other  method 
of  tunneling  commonly  employed  under  similar  conditions. 
This  disadvantage  is  overcome  to  a  considerable  extent  when 
the  tunnel  is  excavated  by  shafts,  and  the  work  at  the  different 
headings  is  so  arranged  that  the  masons  or  excavators  when 
freed  from  duty  at  one  heading  may  be  transferred  to  another 
where  excavation  or  lining  is  to  .be  done  as  the  case  may  be. 
Another  disadvantage  of  the  English  method  arises  from  the 
excavation  of  the  full  section  at  once,  which  in  unstable  soils 
necessitates  strong  and  careful  strutting,  and  increases  the 
danger  of  caving.  The  fact  also  that  the  arch  ring  has  to 
carry  the  weight  of  the  crown  bars,  and  their  loading  at  one 
end  while  the  masonry  is  green,  increases  the  chances  of  the 
arch  being  distorted. 

Conclusion.  —  The  English  method  of  tunneling  in  its  entirety 
is  confined  in  actual  practice  pretty  closely  to  the  country  from 
which  it  receives  its  name.  A  possible  extension  of  its  use 
more  generally  is  considered  by  many  as  likely  to  follow  the 
development  of  a  successful  excavating  machine  for  soft 
material.  The  space  afforded  by  the  opening  of  the  full  sec- 
tion at  once,  especially  adapts  the  method  to  the  use  of  exca- 


162 


TUNNELING 


vators  like,  for  example,  the  endless  chain  bucket  excavator 
used  on  the  Central  London  Ry.,  and  illustrated  in  Fig.  12. 
The  method  also  furnishes  an  excellent  opportunity  for  electric 
hauling  and  lighting  during  construction. 

The  English  method  of  tunneling  has  been  used  in  building 
the  Hoosac,  Musconetcong,  Allegheny,  Baltimore  and  Potomac, 
and  other  tunnels  in  America.  The  names  of  the  European 
tunnels  built  by  this  method  are  too  numerous  to  mention  here. 


AUSTRIAN  METHOD. 

The  Austrian  full-section  method  of  tunneling  through  soft 
ground  was  first  used  in  constructing  the  Oberau  tunnel  on  the 
Leipsic  and  Dresden  R.R.,  in  Austria  in  1837.  It  consists  in 
excavating  the  full  section  and  building  up  the  lining  masonry 
from  the  foundations  as  in  the  English,  but  with  the  important 
exception  that  the  invert  is  built  last  instead  of  first  in  all  cases 
except  where  the  presence  of  very  loose  soil  requires  its  con- 
struction first.  A  still  more  important  difference  in  the  two 
methods  is  that  the  excavation  is  carried  out  in  smaller  sections 
and  is  continuous  in  the  Austrian  method  instead  of  alternating 
with  the  mason  work  as  it  does  in  the  English  method. 


FlGS.  86  and  87.  —  Diagrams  Showing  Sequence  of  Excavation  in  Austrian  Method 
of  Tunneling. 

Excavation.  —  The  excavation  in  the  Austrian  method  begins 
by  driving  the  bottom  center  drift  No.  1,  Fig.  86,  rising  from 
the  floor  of  the  tunnel  section  nearly  to  the  height  of  the 


THE   FULL   SECTION    METHOD  163 

springing  lines  of  the  roof  arch.  When  this  drift  has  been 
driven  ahead  a  distance  varying  from  12  ft.  to  20  ft.  or  some- 
times more,  the  excavation  of  the  center  top  heading  No.  2  is 
driven  for  the  same  distance.  The  next  operation  is  to  retiove 
part  No.  3,  thus  forming  a  central  passage  the  full  depth  of  the 
tunnel  section  at  the  center.  This  trench  is  enlarged  by 
removing  parts  Nos.  4,  5,  6,  7,  and  8  in  the  order  named  until 
the  full  section  is  opened.  A  modification  of  this  plan  of 
excavation  is  shown  by  Fig.  87  which  is  used  in  firm  soils. 

Strutting.  —  Each  part  of  the  section  is  strutted  as  fast  as 
it  is  excavated.  The  center  bottom  drift  first  excavated  is 
strutted  by  laying  a  transverse  sill  across  the  floor,  raising 
two  side  posts  from  it,  and  capping  them  with  a  transverse 
timber  having  its  ends  projecting  beyond  the  side  posts  and 
halved  as  shown  by  Fig.  88.  The  top  center  heading  No.  2, 
which  is  next  excavated,  is  strutted  by  means  of  two  side  posts 
resting  on  blocking  and  carrying  a  transverse  cap  as  also  shown 
by  Fig.  88.  Sometimes  the  side  posts  in  the  heading  strutting- 
frames  are  also  carried  on  a  transverse  sill  as  are  those  of  the 
bottom  drift.  This  construction  is  usually  adopted  in  loose 
soils.  When  the  sill  is  employed,  the  middle  part,  No.  3,  is 
strutted  by  inserting  side  posts  between  the  bottom  of  the  top 
sill  and  the  cap  of  the  frame  in  the  drift  below.  When,  how- 
ever, the  posts  of  the  top  heading  frame  are  carried  on  blocking, 
it  is  the  practice  to  replace  them  with  long  posts  rising  from 
the  cap  of  the  bottom  drift  frame  to  the  cap  of  the  top  heading 
frame.  Further,  when  the  intermediate  sill  is  employed  at  the 
bottom  level  of  the  top  heading  it  projects  beyond  the  side 
posts  and  has  its  ends  halved. 

After  the  completion  of  the  center  trench  strutting  the  next 
task  is  to  strut  parts  Nos.  4  and  5.  This  is  done  by  continuing 
the  upper  sill  by  means  of  a  timber  having  one  end  halved  to 
join  with  the  projecting  end  of  the  sill  in  position.  This  ex- 
tension timber  is  shown  at  a,  Fig.  89.  The  next  operation  is 
to  place  the  timber  5,  having  one  end  resting  on  the  cap-piece 


164 


TUNNELING 


of  the  top  heading  frame  and  the  other  beveled  and  resting  on 
the  top  of  the  sill  a  near  the  end.  The  timber  b  is  laid  tangent 
to  the  curve  of  the  roof  arch,  and  to  support  it  against  flexure 
the  strut  c  is  inserted  as  shown.  To  support  the  thrust  of  this 


strut  the  additional  post  d  is 
inserted  and  the  original  bot- 
tom heading  frame  is  rein- 
forced as  shown.  The  next 
step  is  to  insert  the  strut  e, 
and  when  this  and  the  previ- 
ous construction  are  dupli- 
cated on  the  opposite  side  of 
the  tunnel  section  we  have 
the  strutting  of  the  parts  Nos. 
1  to  5,  inclusive,  complete. 
Part  No.  6  is  then  removed 
and  strutted  by  extending  the 
bottom  drift  cap-piece  by  a 

timber  similar  to  timber  a  above,  and  then  by  inserting  a  side 
strut  between  the  outer  ends  of  these  two  timbers,  as  indicated 
by  Fig.  90.  As  the  final  parts,  Nos.  7  and  8,  are  removed,  the 
inclined  prop  a,  Fig.  90,  is  inserted  as  shown.  When  the  soil 


FlGS.  88  to  90.  —  Sketches  Showing  Construc- 
tion of  Strutting,  Austrian  Method. 


THE    FULL   SECTION   METHOD 


165 


is  loose  some  of  the  members  of  the  framework  are  doubled 
and  additional  bracing  is  introduced  as  shown  by  Fig.  90. 

The  frames  just  described  are  placed  at  intervals  of  about 
4  ft.  along  the  excavation,  and  are  braced  apart  by  horizontal 
struts.  Some  of  the  longitudinal  bearing  beams,  as  at  5,  Fig. 
90,  also  extend  through  two  or  three  frames,  and  help  to  tie 
them  together.  Finally,  the  longitudinal  poling-boards  extend- 
ing from  one  frame  to  the  next  along  the  walls  of  the  excava- 
tion serve  to  connect  them  together.  The  short  transverse 
beam  e,  Fig.  90,  located  just  above  the  floor  of  the  invert, 
serves  to  carry  the  planking  upon  which  the  train  car  tracks 
are  laid.  Besides  the  timber  strutting  peculiar  to  the  Austrian 
method,  the  Rziha  iron  strutting  described  in  a  previous  chapter 
is  frequently  used  in  tunneling  by  the  Austrian  process. 

Centers.  —  The  two  forms  of  centers  used  in  the  English 
method  of  tunneling  are  also 
used  in  the  Austrian  method. 
One  of  the  methods  of  support- 
ing these  centers  is  shown  by 
Fig.  91.  The  tie-beam  of  the 
tenter  rests  on  longitudinal  tim- 
bers carried  by  the  strutting 
frames  and  intermediate  props. 
In  single-track  tunnels  it  is  the 
frequent  practice  also  to  carry 
the  ends  of  the  tie-beams  in  re- 
cesses left  in  the  side  wall  ma- 
sonry, with  intermediate  props 
inserted  to  prevent  flexure  at 
the  center.  When  the  Rziha 
iron  strutting  is  employed,  it  also 
serves  for  the  centering  upon  which  the  arch  masonry  is  built. 

Masonry.  —  In  the  Austrian  system  of  tunneling,  the  lining 
is  built  from  the  foundations  of  the  side  walls  upward  to  the 
crown  of  the  roof  arch  in  lengths  in  consecutive  rings  equal  to 


FIG.  91.  —  Sketch  Showing  Manner  of 
Constructing  the  Lining  Masonry, 
Austrian  Method. 


166  TUNNELING 

the  lengths  of  the  consecutive  openings  of  the  full  section,  or 
from  12  ft.  to  20  ft.  long.  Except  in  infrequent  cases  in  very 
loose  materials  the  invert  is  the  last  part  of  the  masonry  to  be 
built,  since  to  build  it  first  requires  the  removal  of  the  strutting 
which  cannot  easily  or  safely  be  accomplished  until  the  side  walls 
and  roof  arch  are  completed.  As  the  side  wall  foundations  are 
built,  however,  their  interior  faces  are  left  inclined,  as  shown 
by  Figs.  90  and  91,  ready  for  the  insertion  of  the  invert,  and 
are  meanwhile  kept  from  sliding  inward  by  the  insertion  of 
blocking  between  them  and  the  bottom  of  the  strutting.  Fig. 
91  shows  the  nature  of  this  blocking,  and  also  the  manner  in 
which  the  side  wall  and  roof  arch  masonry  is  carried  upward. 
Finally  when  the  roof  arch  is  keyed  and  the  centers  are  struck, 
the  strutting  is  taken  down  and  the  invert  is  built. 

Advantages  and  Disadvantages.  —  The  principal  advantages 
claimed  for  the  Austrian  method  of  tunneling  are :  (1)  The 
excavation  being  conducted  by  driving  a  large  number  of  con- 
secutive small  galleries,  which  are  immediately  strutted,  there 
is  little  disturbance  of  the  surrounding  material ;  (2)  the 
polygonal  type  of  strutting  adopted  is  easily  erected  and  of 
great  strength  against  symmetrical  pressures  ;  (3)  the  masonry, 
being  built  from  the  foundations  up,  is  a  single  homogeneous 
structure,  and  is  thus  better  able  to  withstand  dangerous  pres- 
sures ;  (4)  the  excavation  is  so  conducted  that  the  masons 
and  excavators  do  not  interfere,  and  both  can  work  at  the  same 
time.  The  disadvantages  which  the  method  possesses  are  :  (1) 
The  strutting,  while  very  strong  under  symmetrical  pressures, 
either  vertical  or  lateral,  is  distorted  easily  by  unsymmetrical 
vertical  or  lateral  pressures,  and  by  pressure  in  the  direction  of 
the  axis  of  the  tunnel;  (2)  the  construction  of  the  invert  last 
exposes  the  side  walls  to  the  danger  of  being  squeezed  together, 
causing  a  rotation  of  the  arch  of  the  nature  discussed  in  de- 
scribing the  Belgian  method  of  tunneling. 


SPECIAL   TKEACHEKOUS    GilOUXD   METHOD  167 


CHAPTER   XVI. 

SPECIAL     TREACHEROUS      GROUND      METHOD; 
ITALIAN   METHOD;   QUICKSAND  TUN- 
NELING;   PILOT    METHOD. 


ITALIAN   METHOD. 

THE  Italian  method  of  tunneling  was  first  employed  in  con- 
structing the  Cristina  tunnel  on  the  Foggia  &  Benevento  R.R. 
in  Italy.  This  tunnel  penetrated  a  laminated  clay  of  the  most 
treacherous  character,  and  after  various  other  soft-ground 
methods  of  tunneling  had  been  tried  and  had  failed,  Mr.  Procke, 
the  engineer,  devised  and  used  successfully  the  method  which 
is  now  known  as  the  Italian  or  Cristina  method.  The  Italian 
method  is  essentially  a  treacherous  soil  method.  It  consists  in 
excavating  the  bottom  half  of  the  section  by  means  of  several 
successive  drifts,  and  building  the  invert  and  side  walls ;  the 
space  is  then  refilled  and  the  upper  half  of  the  section  is  exca- 
vated, and  the  remainder  of  the  side  walls  and  the  roof  arch 
are  built ;  finally,  the  earth  filling  in  the  lower  half  of  the 
section  is  re-excavated  and  the  tunnel  completed.  The  method 
is  an  expensive  one,  but  it  has  proved  remarkably  successful  in 
treacherous  soils  such  as  those  of  the  Apennine  Mountains, 
in  which  some  of  the  most  notable  Italian  tunnels  are  located. 
It  is,  moreover,  a  single-track  tunnel  method,  since  any  soil 
which  is  so  treacherous  as  to  warrant  its  use  is  too  treacherous 
to  permit  an  opening  to  be  excavated  of  sufficient  size  for  a 
double-track  railway,  except  by  the  use  of  shields. 

Excavation.  —  The  plan  of  excavation  in  the  Italian  method 
is  shown  by  the  diagram  Fig.  92.  Work  is  begun  by  driving 


168 


TUNNELING 


the  center  bottom  heading  No.  1,  and  this  is  widened  by  taking 
out  parts  No.  2.  Finally  part  No.  3  is  removed,  and  the  lower 
half  of  the  section  is  open.  As  soon  as  the  invert  and  side 
wall  masonry  has  been  built  in  this  excavation,  parts  No.  2 
are  filled  in  again  with  earth.  The  exca- 
vation of  the  center  top  heading  No.  4  is 
then  begun,  and  is  enlarged  by  removing 
the  earth  of  part  No.  5.  The  faces  of  this 
last  part  are  inclined  so  as  to  reduce  their 
tendency  to  slide,  and  to  permit  of  a 
greater  number  of  radial  struts  to  be 
placed.  Next,  parts  No.  6  are  excavated, 
and  when  this  is  done  the  entire  section, 
except  for  the  thin  strip  No.  7,  has  been 
opened.  At  the  ends  of  part  No.  7  nar- 
sunk  to  reach  the  tops  of  the  side  walls 
in  the  lower  half  of  the  section.  The 


FIG.  92.  — Diagram  Show- 
ing Sequence  of  Excava- 
tion in  Italian  Method  of 
Tunneling. 


row  trenches  are 
already  constructed 
masonry  is  then  completed  for  the  upper  half  of  the  section, 
and  part  No.  7  and  the  filling  in  parts  No.  2  are  removed. 
The  various  drifts  and  headings  and  ^ — -^ 

the  parts  excavated  to  enlarge  them  NX 

are  seldom  excavated  more  than  from         /  \ 

6  ft.  to  10  ft.  ahead  of  the  lining. 

Strutting.  -  -  The  bottom  center 
drift,  which  is  first  driven,  is  strutted 
by  means  of  frames  consisting  of  side 
posts  resting  on  floor  blocks  and  car- 
rying a  cap-piece.  Poling-boards  are 
placed  around  the  walls,  stretching 
from  one  frame  to  the  next.  As 
soon  as  the  invert  is  sufficiently  completed  to  permit  it,  the 
side  posts  of  the  strutting  frames  are  replaced  by  short  struts 
resting  on  the  invert  masonry  as  shown  by  Fig.  93.  To  permit 
the  old  side  posts  to  be  removed  and  the  new  shorter  ones  to 
be  inserted,  the  cap-piece  of  the  frame  is  temporarily  supported 


FIG.  93.  — Sketch  Showing  Strut- 
ting for  Lower  Part  of  Section. 


SPECIAL   TREACHEROUS    GROUND   METHOD  169 

by  inclined  props  arranged  as  shown  by  Fig.  97.  When  parts 
No.  2  are  excavated  the  roof  is  strutted  by  inserting  the  trans- 
verse caps  a,  Fig.  93,  the  outer  ends  of  which  are  carriedjby  the 
system  of  struts  £>,  c,  J,  and  e.  The  longitudinal  poling-boards 
supporting  the  ceiling  and  walls  are  held  in  place  by  the  cap 
a  and  the  side  timber  e.  To  stiffen  the  frames  longitudinally 
of  the  tunnel,  horizontal  longitudinal  struts  are  inserted  between 
them. 

The  excavation  of  the  upper  half  of  the  tunnel  section  is 
strutted  as  in  the  Belgian  method,  with  radial  struts  carrying 
longitudinal  roof  bars  and  transverse  poling-boards.  On  ac- 
count of  the  enormous  pressures  developed  by  the  treacherous 
soils  in  which  only  is  the  Italian  method  employed,  the  radial 
strutting  frames  and  crown  bars  must  be  of  great  strength, 


FIGS.  94  and  95.  —  Sketches  Showing  Construction  of  Centers,  Italian  Method. 

while  the  successive  frames  must  be  placed  at  frequent  intervals, 
usually  not  more  than  3  ft.  After  the  masonry  side  walls  have 
been  built  in  the  lower  part  of  the  excavation,  longitudinal 
planks  are  laid  against  the  side  posts  of  the  center  bottom 
drift  frames,  to  form  an  enclosure  for  the  filling-in  of  parts 
No.  2.  The  object  of  this  filling  is  principally  to  prevent 
the  squeezing-in  of  the  side  walls. 

Centers.  —  Owing  to  the  great  pressures  to  be  resisted  in  the 
treacherous  soils  in  which  the  Italian  method  is  used,  the  con- 
struction of  the  centers  has  to  be  very  strong  and  rigid.  Figs. 
94  and  95  show  two  common  types  of  center  construction  used 
with  this  method.  The  construction  shown  in  Fig.  94  is  a 
strong  one  where  only  pressures  normal  to  the  axis  of  the 
tunnel  have  to  be  withstood,  but  it  is  likely  to  twist  under 


170  TUNNELING 

pressures  parallel  to  the  axis  of  the  tunnel.  In  the  construc- 
tion shown  by  Fig.  95,  special  provision  is  made  to  resist 
pressures  normal  to  the  plane  of  the  center  or  twisting  pres- 
sures, by  the  strength  of  the  transverse  bracing  extending  hori- 
zontally across  the  center. 

Masonry,  —  The  construction  of  the  masonry  lining  begins 
with  the  invert,  as  indicated  by  Fig.  93,  and  is  carried  up  to  the 
roof  of  parts  No.  2,  as  already  indicated,  and  is  then  discon- 
tinued until  the  upper  parts  Nos.  4,  5,  and  6  are  excavated. 
The  next  step  is  to  sink  side  trenches  at  the  ends  of  part  No.  7,. 
which  reach  to  the  top  of  the  completed  side  walls.  This 
operation  leaves  the  way  clear  to  finish  the  side  walls  and  to 
construct  the  roof  arch  in  the  ordinary  manner  of  such  work  in 

tunneling.  Since  this  method  of 
tunneling  is  used  only  in  very  soft 
ground  which  yields  under  load,  the 
usual  practice  is  to  construct  the  in- 
vert and  side  walls  on  a  continuous 
no.  96. -sketch  showing  invert  foundation  course  of  concrete  as  in- 

and  Foundation  Masonry,  Italian      dicated    by    Fig.    96.       The    lining    is 
Method.  ..      .      .,      .  .  , 

usually  built  in  successive  rings,  and 

the  usual  precautions  are  taken  with  respect  to  filling  in  the 
voids  behind  the  lining.  The  thickness  of  the  lining  is  based 
upon  the  figures  for  laminated  clay  of  the  third  variety  given 
in  Table  II. 

Hauling The  system  of  hauling  adopted  with  this  method 

of  tunneling  is  very  simple,  since  the  excavation  of  the  various 
parts  is  driven  only  from  6  ft.  to  10  ft.  ahead,  and  the  work  pro- 
gresses slowly  to  allow  for  the  construction  of  the  heavy  strutting 
required.  To  take  away  the  material  from  the  center  bottom 
drift,  narrow-gauge  tracks  carried  by  cross-beams  between  the 
side  posts  above  the  floor  line  are  employed.  This  same 
narrow-gauge  line  is  employed  to  take  away  a  portion  of  parts 
No.  2,  the  remaining  portion  being  left  and  used  for  the  refill- 
ing after  the  bottom  portion  of  the  lining  has  been  built,  as 


SPECIAL   TREACHEROUS   GROUND   METHOD 


171 


previously  described.  The  upper  half  of  the  section  being  ex- 
cavated, as  in  the  Belgian  method,  the  system  of  hauling  with 
inclined  planes  to  the  tunnel  floor  below,  which  is  a  character- 
istic of  that  method,  may  be  employed.  It  is  the  more  usual 


FIG.  97.  —  Sketch  Showing  Longitudinal  Section  of  a  Tunnel  under  Construction, 
Italian  Method. 

practice,  however,  since  the  excavation  is  carried  so  little  a  dis- 
tance ahead  and  progresses  so  slowly,  to  handle  the  spoil  from 
the  upper  part  of  the  section  by  wheelbarrows  which  dump  it 
into  the  cars  running  on  the  tunnel  floor  below.  Hand  labor 
is  also  used  to  raise  the  construction  ma- 
terials used  in  excavating  the  upper  sec- 
tion. The  tracks  on  the  tunnel  floor, 
besides  extending  to  the  front  of  the  ad- 
vanced bottom  center  drift,  have  right  and 
left  switches  to  be  employed  in  removing 
the  refilling  in  parts  No.  2,  the  spoil  from 
the  upper  part  of  the  section,  and  the 
material  of  part  No.  7.  Fig.  97  is  a  longi- 
tudinal section  showing  the  plan  of  exca- 
vation and  strutting  adopted  with  the  Italian  method. 

Modifications.  —  It  often  happens  that  the  filling  placed  be- 
tween the  side  walls  and  the  planking,  which  is  practically  the 
space  comprised  by  parts  No.  2,  is  not  sufficient  to  resist  the 
inward  pressure  of  the  walls,  and  they  tip  inward.  In  these 
cases  a  common  expedient  is  to  substitute  for  the  earth  filling 


98.  —  Sketch  Showing 
Sequence  of  Excavation, 
Stazza  Tunnel. 


172 


TUNNELING 


a   temporary   masonry   arch    sprung    between    the    side   walls 
with  its  feet  near  the  bottom  of  the   walls,   and  its  crown, 
just  below  the   level  of   their  tops,    as   shown   by  Fig.  101. 
This    construction    was    employed  in  the 
Stazza   tunnel    in   Italy.     In    this  tunnel 
the  excavation  was  begun  by  driving  the 
center  drift,  No.  1,  Fig.  98,  and  immedi- 
ately strutting   it   as    shown  by  Fig.  99. 
The  other  parts,  Nos.  2  and  3,  completing 
the  lower  portion  of  the  section,  were  then 
taken  out  and  strutted.     While  part  No.  2 
PIG.  99.— sketch  stowing   was  being  excavated  at  the  bottom,  and 
Drif^st°afzzarT^neiFir  '    tne   center   part  of   the  invert  built,  the 
longitudinal  crown  bars  carrying  the  roof 
of   the    excavation  were  carried   temporarily  by   the  inclined 
props  shown  by  Fig.  100.     After  completing  the  invert  and 
the  side  walls  to  a  height  of  2  or  3  ft.,  a  thick  masonry  arch 
was  sprung  between  the  side  walls,  as  shown  in  transverse 
section   by  Fig.  101,  and  in  longitudinal  section   by  Fig.  100. 
This  arch  braced  the  side  walls  against  tipping  inward,  and 


FIGS.  100  and  101.—  Sketches  Showing  Temporary  Strutting  Arch  Construction, 
Stazza  Tunnel. 

•carried  short  struts  to  support  the  crown  bars.  The  haunches 
of  the  arch  were  also  filled  in  with  rammed  earth.  The  upper 
half  of  the  section  was  excavated,  strutted,  and  lined  as  in 
the  standard  Italian  method  previously  described.  When  the 
lining  was  completed,  the  arch  inserted  between  the  side  walls 
was  broken  down  and  removed. 


SPECIAL   TREACHEROUS   GROUND   METHOD  17$ 

Advantages  and  Disadvantages.  —  The  great  advantage  claimed 
for  the  Italian  method  of  tunneling  is  that  it  is  built  in  two- 
separate  parts,  each  of  which  is  separately  excavated,  strutted, 
and  lined,  and  thus  can  be  employed  successfully  in  very 
treacherous  soils.  Its  chief  disadvantage  is  its  excessive  cost, 
which  limits  its  use  to  tunnels  through  treacherous  soils  where 
other  methods  of  timbering  cannot  be  used. 

QUICKSAND   TUTOELING. 

When  an  underground  stream  of  water  passes  with  force 
through  a  bed  of  sand  it  produces  the  phenomenon  known  a& 
quicksand.  This  phenomenon  is  due  to  the  fineness  of  the 
particles  of  sand  and  to  the  force  of  the  water,  and  its  activity 
is  directly  proportional  to  them.  When  sand  is  confined  it 
furnishes  a  good  foundation  bed,  since  it  is  practically  incom- 
pressible. To  work  successfully  in  quicksand,  therefore,  it  is 
necessary  to  drain  it  and  to  confine  the  particles  of  sand  so 
that  they  cannot  flow  away  with  the  water.  This  observation 
suggests  the  mode  of  procedure  adopted  in  excavating  tunnels 
through  quicksand,  which  is  to  drain  the  tunnel  section  by 
opening  a  gallery  at  its  bottom  to  collect  and  carry  away  the 
water,  and  to  prevent  the  movement  or  flowing  of  the  sand  by 
strutting  the  sides  of  the  excavation  with  a  tight  planking. 

The  sand  having  to  be  drained  and  confined  as  described,  the 
ordinary  methods  of  soft-ground  tunneling  must  be  employed, 
with  the  following  modifications : 

(1)  The  first  work  to  be  performed  is  to  open  a  bottom 
gallery  to  drain  the  tunnel.     This  gallery  should  be  lined  with 
boards  laid  close  and  braced  sufficiently  by  interior  frames  to 
prevent  distortion  of  the  lining.     The  interstices  or  seams  be- 
tween the  lining  boards  snould  be  packed  with  straw  so  as  to 
permit  the  percolation  of  water  and  }ret  prevent  the  movement 
of  the  sand. 

(2)  As  fast  as  the  excavation  progresses  its  walls  should 


174  TUNNELING 

be  strutted  by  planks  laid  close,  and  held  in  position  by  interior 
framework;  the  seams  between  the  plank  should  be  packed 
with  straw. 

(8)  The  masonry  Lining  should  be  built  in  successive  rings, 
and  the  work  so  arranged  that  the  water  seeping  in  at  the  sides 
and  roof  is  collected  and  removed  from  the  tunnel  immediately. 

Excavation.  —  The  best  and  most  commonly  employed  method 
of  driving  tunnels  through  quicksand  is  a  modification  of  the 
Belgian  method.  At  first  sight  it  may  appear  a  hazardous  work 
to  support  the  roof  arch,  as  is  the  characteristic  of  this  method, 
on  the  unexcavated  soil  below,  when  this  soil  is  quicksand,  but 
if  the  sand  is  well  confined  and  drained  the  risk  is  really  not 
very  great.  Next  to  the  Belgian  method  the  German  method 
is  perhaps  the  best  for  tunneling  quicksand.  In  these  compari- 
sons the  shield  system  of  tunneling  is  for  the  time  being  left 
out  of  consideration.  This  method  will  be  described  in  suc- 
ceeding chapters.  Whenever  any  of  the  systems  of  tunneling 
previously  described  are  employed,  the  first  task  is  always  to 
open  a  drainage  gallery  at  the  bottom  of  the  section. 

Assuming  the  Belgian  method  is  to  be  the  one  adopted,  the 
first  work  is  to  drive  a  center  bottom  drift,  the  floor  of  which 
is  at  the  level  of  the  extrados  of  the  invert.  This  drift  is  im- 
mediately strutted  by  successive  transverse  frames  made  up  of 
a  sill,  side  posts,  and  a  cap  which  support  a  close  plank  strut- 
ting or  lining,  with  its  joints  packed  with  straw.  Between  the 
side  posts  of  each  cross-frame,  at  about  the  height  of  the 
intrados  of  the  invert,  a  cross-beam  is  placed  ;  and  on  these  cross- 
beams a  plank  flooring  is  laid,  which  divides  the  drift  horizon- 
tally into  two  sections,  as  shown  by  Fig.  102;  the  lower  section 
forming  a  covered  drain  for  the  seepage  water,  and  the  upper 
providing  a  passageway  for  workmen  and  cars.  The  bottom 
drift  is  driven  as  far  ahead  as  practicable,  in  order  to  drain  the 
sand  for  as  great  a  distance  in  advance  of  the  work  as  possible. 
After  the  construction  of  the  bottom  drainage  drift  the  excava- 
tion proper  is  begun,  as  it  ordinarily  is  in  the  Belgian  method 


SPECIAL   TREACHEROUS    GROUND    METHOD 


175 


FIG.  102.— Sketch  Showing 
Preliminary  Drainage  Gal- 
leries, Quicksand  Method. 


by  driving  a  top  center  heading,  as  shown  by  Fig.  102.  This 
heading  is  deepened  and  widened  after  the  manner  usual  to  the 
Belgian  method,  until  the  top  of  the  sec- 
tion is  open  down  to  the  springing  lines 
of  the  roof  arch.  To  collect  the  seepage 
water  from  the  center  top  heading  it  is 
provided  with  a  center  bottom  drain  con- 
structed like  the  drain  in  the  bottom 
drift,  as  shown  by  Fig.  102.  When  the 
top  heading  is  deepened  to  the  level  of 
the  springing  lines  of  the  roof  arch,  its 
bottom  drain  is  reconstructed  at  the  new 
level,  and  serves  to  drain  the  full  top 
section  opened  for  the  construction  of  the 
roof  arch.  This  top  drain  is  usually  con- 
structed to  empty  into  the  drain  in  the  bottom  drift. 

Strutting.  —  The  method  of  strutting  the  bottom  drift  has 
already  been  described.  For  the  remainder  of  the  excavation 
the  regular  Belgian  method  of  radial  roof  strutting-frames  is 

employed,  as  shown  by  Fig.  103. 
Contrary  to  what  might  be  expected, 
the  number  of  radial  struts  required 
is  not  usually  greater  than  would  be 
used  in  many  other  soils  besides 
quicksand.  Single-track  railway  tun- 
nels have  been  constructed  through 
quicksand  in  several  instances  where 
the  number  of  radial  props  required 
on  each  side  of  the  center  did  not 
exceed  four  or  five.  It  is  necessary, 
however,  to  place  the  poling-boards 

very  close  together,  and  to  pack  the  joints  between  them  to 
prevent  the  inflow  of  the  fine  sand.  In  strutting  the  lower 
part  of  the  section  it  is  also  necessary  to  support  the  sides  with 
tight  planking.  This  is  usually  held  in  place  by  longitudinal 


FIG.  103.  —  Sketch  Showing  Con- 
struction of  Roof  Strutting 
Quicksand  Method. 


176 


TUNNELING 


FIG.  104.  —  Sketch  Showing  Construc- 
tion of  Masonry  Lining,  Quicksand 
Method. 


bars  braced  by  short  struts  against  the  inclined  props  employed 
to  carry  the  roof  arch  when  the  material  on  which  they  origi- 
nally rested  is  removed.    This  side 
strutting   is    shown    at    the    right 
hand  of  Fig.  104. 

Masonry.  — As  soon  as  the  upper 
part  of  the  section  has  been  opened 
the  roof  arch  is  built  with  its  feet 
resting  on  planks  laid  on  the  unex- 
cavated  material  below.  This  arch 
is  builjb  exactly  as  in  the  regular 
Belgian  method  previously  de- 
scribed, using  the  same  forms  of 
centers  and  the  same  methods 
throughout,  except  that  the  poling- 
boards  of  the  strutting  are  usually  left  remaining  above  the 
arch  masonry.  To  prevent  the  possibility  of  water  percolating 
through  the  arch  masonry,  many  engineers  also  advise  the 
plastering  of  the  extrados  of  the  arch  with  a  layer  of  cement 
mortar.  This  plastering  is  designed  to  lead  the  water  along 
the  haunches  of  the  arch  and  down  behind  the  side  walls.  In 
constructing  the  masonry  below  the  roof  arch  the  invert  is 
built  first,  contrary  to  the  regular  Belgian  method,  and  the 
side  walls  are  carried  up  on  each  side  from  the  invert  ma- 
sonry. Seepage  holes  are  left  in  the  invert  masonry,  and  also 
in  the  side  Avails  just  above  the  intrados  of  the  invert.  At  the 
center  of  the  invert  a  culvert  or  drain  is  constructed,  as  shown 
by  Fig.  104,  inside  the  invert  masonry.  This  culvert  is  com- 
monly made  with  an  elliptical  section  with  its  major  axis  hori- 
zontal, and  having  openings  at  frequent  intervals  at  its  top. 
The  thickness  of  the  lining  masonry  required  in  quicksand  is 
shown  by  Table  II. 

Removing  the  Seepage  Water.  —  After  the  tunnel  is  completed 
the  water  which  seeps  in  through  the  weep-holes  left  in  the  ma- 
sonry passes  out  of  the  tunnel,  following  the  direction  of  the 


SPECIAL,   TREACHEROUS   GROUND  METHOD  177 

descending  grades.  During  construction,  however,  special 
means  will  have  to  be  provided  for  removing  the  water  from 
the  excavation,  their  character  depending  upon  the  method  of 
excavation  and  upon  the  grades  of  the  tunnel  bottom.  When 
the  excavation  is  carried  on  from  the  entrances  only,  unless  the 
tunnel  has  a  descending  grade  from  the  center  toward  each  end, 
the  tunnel  floor  in  one  heading  will  be  below  the  level  of  the  en- 
trance, or,  in  other  words,  the  descending  grade  will  be  toward 
the  point  where  work  is  going  on,  while  at  the  opposite  entrance 
the  grade  will  be  descending  from  the  work.  In  the  latter 
case  the  removal  of  the  seepage  water  is  easily  accomplished  by 
means  of  a  drainage  channel  along  the  bottom  of  the  excavation. 
In  the  former  case  the  water  which  drains  toward  the  front  is 
collected  in  a  sump,  and  if  there  is  not  too  great  a  difference  in 
level  between  this  sump  and  the  entrance,  a  siphon  may  be  used 
to  remove  it.  Where  the  siphon  cannot  be  used,  pumps  are 
installed  to  remove  the  water.  When  the  tunnel  is  excavated 
by  shafts  the  condition  of  one  high  and  one  low  front,  as  com- 
pared with  the  level  at  the  shaft,  is  had  at  each  shaft.  Gene- 
rally, therefore,  a  sump  is  constructed  at  the  bottom  of  the 
shaft ;  the  culvert  from  the  high  front  drains  directly  to  the 
shaft  sump,  while  the  water  from  the  low-front  sump  is  either 
siphoned  or  pumped  to  the  shaft  sump.  From  the  shaft  sump 
the  water  is  forced  up  the  shaft  to  the  surface  by  pumps. 

THE  PHOT  METHOD. 

The  pilot  system  of  tunneling  has  been  successfully  em- 
ployed in  constructing  soft-ground  sewer  tunnels  in  America 
by  the  firm  of  Anderson  &  Barr,  which  controls  the  patents. 
The  most  important  work  on  which  the  system  has  been  em- 
ployed is  the  main  relief  sewer  tunnel  built  in  Brooklyn,  N.Y., 
in  1892.  This  work  comprised  800  ft  of  circular  tunnel  15  ft. 
in  diameter,  4400  ft  14  ft.  in  diameter,  3200  ft  12  ft.  in 
diameter,  and  1000  ft.  10  ft.  in  diameter,  or  9400  ft.  of  tunnel 


178 


TUNNELING 


altogether.     The  method  of  construction  by  the  pilot  system  is 
as  follows : 

Shafts  large  enough  for  the  proper  conveyance  of  materials 
from  and  into  the  tunnel  are  sunk  at  such  places  on  the  line  of 
work  as  are  most  convenient  for  the  purpose.  From  these 
shafts  a  small  tunnel,  technically  a  pilot,  about  6  ft.  in  diameter, 
composed  of  rolled  boiler  iron  riveted  to  light  angle  irons  on 
four  sides,  perforated  for  bolts,  and  bent  to  the  required  radius 
of  the  pilot,  is  built  into  the  central  part  of  the  excavation  on 
the  axis  of  the  tunnel.  This  pilot  is  generally  kept  about  30  ft. 
in  advance  of  the  completed  excavation,  as  shown  by  Fig.  105. 
The  material  around  the  exterior  of  the  pilot  is  then  excavated, 
using  the  pilot  as  a  support  for  braces  which  radiate  from  it  and 


Bracing.'" v  Arch  Constriction. 

FIG.  105.  —  Sketch  Showing  Pilot  Method  of  Tunneling. 

secure  in  position  the  plates  of  the  outside  shell  which  holds 
the  sand,  gravel,  or  other  material  in  place  until  the  concentric 
rings  of  brick  masonry  are  built.  Ribs  of  T-iron  bent  to  the 
radius  of  the  interior  of  the  brick  work,  and  supported  by  the 
braces  radiating  from  the  pilot,  are  used  as  centering  supports 
for  the  masonry.  On  these  ribs  narrow  lagging-boards  are  laid 
as  the  construction  of  the  arch  proceeds,  the  braces  holding  the 
shell  plates  and  the  superincumbent  mass  being  removed  as  the 
masonry  progresses.  The  key  bricks  of  the  arches  are  placed 
in  position  on  ingeniously  contrived  key-boards,  about  12  ins.  in 
width,  which  are  fitted  into  rabbeted  lagging-boards  one  after 
another  as  the  key  bricks  are  laid  in  place.  After  the  masonry 
has  been  in  place  at  least  twenty-four  hours,  allowing  the  cement 


SPECIAL   TREACHEROUS   GROUND   METHOD  179 

mortar  time  to  set,  the  braces,  ribs,  and  lagging  which  support 
it  are  removed.  In  the  meantime  the  excavation,  bracing,  pilot, 
and  exterior  shell  have  been  carried  forward,  preparing  the  way 
for  more  masonry.  The  top  plates  of  the  shell  are  first  placed 
in  position,  the  material  being  excavated  in  advance  and  sup- 
ported by  light  poling-boards ;  then  the  side-plates  are  butted 
to  the  top  and  the  adjoining  side-plates.  In  the  pilot  the  plates 
are  united  continuously  around  the  perimeter  of  the  circle, 
while  in  the  exterior  shell  the  plates  are  used  for  about  one- 
third  of  the  perimeter  on  top,  unless  treacherous  material  is 
encountered,  when  the  plates  are  continued  down  to  the  spring- 
ing lines  of  the  arch.  This  iron  lining  is  left  in  place.  The 
bottom  is  excavated  so  as  to  conform  to  the  exterior  lines  of 
the  masonry.  The  excavation  follows  so  closely  to  the  outer 
lines  of  the  normal  section  of  the  tunnel  that  very  little  loss 
occurs,  even  in  bad  material ;  and  there  is  no  loss  where  suffi- 
cient bond  exists  in  the  material  to  hold  it  in  place  until  the 
poling-boards  are  in  position. 

In  the  Brooklyn  sewer  tunnel  work,  previously  mentioned, 
the  pilot  was  built  of  steel  plates  f  in.  thick,  12  ins.  wide,  and 
37£  ins.  long,  rolled  to  a  radius  of  3  ft.  Steel  angles  4  x  4£  ins. 
were  riveted  along  all  four  sides  of  each  plate,  and  the  plates 
were  bolted  together  by  f -in.  machine-bolts.  The  plates  weighed 
136  Ibs.  each,  and  six  of  them  were  required  to  make  one  com- 
plete ring  6  ft.  in  diameter.  In  bolting  them  together,  iron 
shims  were  placed  between  the  horizontal  joints  to  form  a 
footing  for  the  wooden  braces  for  the  shell,  which  radiate  from 
the  pilot.  The  shell  plates  of  the  15-ft.  section  of  the  tunnel 
were  of  No.  10  steel  12  ins.  wide  and  37  ins.  long,  with  steel 
angles  2^  x  2^  x  f  ins.,  riveted  around  the  edges  the  same  as  for 
the  pilot,  and  put  together  with  |-in.  bolts.  These  plates 
weighed  61  Ibs.  each,  and  eighteen  of  them  were  required  to 
make  one  complete  ring  15  ft.  in  diameter.  The  plates  for  the 
12-ft.  section  were  No.  12  steel  12  ins.  wide  with  2x  2xi-in. 
angles.  Seventeen  plates  were  required  to  make  a  complete  ring. 


180  TUNNELING 


CHAPTER   XVII. 

OPEN-CUT    TUNNELING     METHODS;     TUNNELS 

UNDER  CITY   STREETS;    BOSTON    SUBWAY 

AND    NEW    YORK    RAPID    TRANSIT. 


OPEN-CUT    TUNNELING. 

WHEN  a  tunnel  or  rapid-transit  subway  has  to  be  constructed 
at  a  small  depth  below  the  surface,  the  excavation  is  generally 
performed  more  economically  by  making  an  open  cut  than  by 
subterranean  tunneling  proper.  The  necessary  condition  of 
small  depth  which  makes  open-cut  tunneling  desirable  is  most 
generally  found  in  constructing  rapid-transit  subways  or  tun- 
nels under  city  streets.  This  fact  introduces  the  chief  difficul- 
ties encountered  in  such  work,  since  the  surface  traffic  makes  it 
necessary  to  obstruct  the  streets  as  little  as  possible,  and  has 
led  to  the  development  of  the  several  special  methods  commonly 
employed  in  performing  it.  These  methods  may  be  classed  as 
follows  :  (1)  The  longitudinal  trench  method,  using  either  a 
single  wide  trench  or  two  narrow  parallel  trenches;  (2)  the 
transverse  trench  method. 

Single  Longitudinal  Trench.  —  The  simplest  manner  by  which 
to  construct  open-cut  tunnels  is  to  open  a  single  cut  or  trench 
the  full  width  of  the  tunnel  masonry.  This  trench  is  strutted 
by  means  of  side  sheetings  of  vertical  planks,  held  in  place  by 
transverse  braces  extending  across  the  trench  and  abutting 
against  longitudinal  timbers  laid  against  the  sheeting  plank. 
The  lining  is  built  in  this  trench,  and  is  then  filled  around  and 
above  with  well-rammed  earth,  after  which  the  surface  of  the 
ground  is  restored.  An  especial  merit  of  the  single  longitudi- 
nal trench  method  of  open-cut  tunneling  is  that  it  permits  the 


OPEN-CUT    TUNNELING    METHODS 


181 


L 


FIG.  106.  — Diagram  Showing  Se- 
quence of  Construction  in  Open- 
Cut  Tunnels. 


construction  of  the  lining  in  a  single  piece  from  the  bottom  up, 
thus  enabling  better  workmanship  and  stronger  construction 
than  when  the  separate  parts  are  built  at  different  times.  The 
great  objection  to  the  method  when  B 

it  is  used  for  building  subways 
under  city  streets  is,  that  it  occupies 
so  much  room  that  the  street  usually 
has  to  be  closed  to  regular  traffic. 
For  this  reason  the  single  longi- 
tudinal trench  method  is  seldom 
employed,  except  in  those  portions 
of  city  subways  which  pass  under 
public  squares  or  parks  where  room 
is  plenty. 

Parallel  Longitudinal  Trenches.— 
The  parallel  longitudinal  trench  method  of  open-cut  tunneling 
consists  in  excavating  two  narrow  parallel  trenches  for  the  side 
walls,  leaving  the  center  core  to  be  removed  after  the  side 
walls  have  been  built.  The  diagram,  Fig.  106,  shows  the 

sequence  of  opera- 
tions in  this  method. 
The  two  trenches  No. 
1  are  first  excavated 
a  little  wider  than  the 
side  wall  masonry, 
and  strutted  as  shown 
by  Fig.  107.  At  the 
bottoms  of  these 
trenches  a  foundation 
course  of  concrete  is 
laid,  as  shown  by  Fig. 
108,  if  the  ground  is 

soft ;  or  the  masonry  is  started  directly  on  the  natural  material, 
if  it  is  rock.  From  the  foundations  the  walls  are  carried  up  to 
the  level  of  the  springing  lines  of  the  roof  arch,  if  an  arch  is 


Fio.  107.  —  Sketch  Showing  Method  of  Timbering  Open- 
Cut  Tunnels,  Double  Parallel  Trench  Method. 


182 


TUNNELING 


used ;  or  to  the  level  of  its  ceiling,  if  a  flat  roof  is  used.  After 
the  completion  of  the  side  walls,  the  portion  of  the  excavation 
shown  at  No.  2,  Fig.  106,  is  removed  a  sufficient  depth  to  en- 
able the  roof  arch  to  be  built.  When  the  arch  is  completed,  it 
is  filled  above  with  well-ram  rned  earth,  and  the  surface  is  re- 
stored. The  excavation  of  part  No.  3  inclosed  by  the  side 
walls  and  roof  arch  is  carried  on  from  the  entrances  and  from 
shafts  left  at  intervals  along  the  line. 

A  modification  of  the  method  just  described  was  employed 
in  constructing  the  Paris  underground  railways.  It  consists  in 
excavating  a  single  longitudinal  trench  along  one  side  of  the 
street,  and  building  the  side  wall  in  it  as  previously  described. 
When  this  side  wall  is  completed  to  the 
roof,  the  right  half  of  part  No.  2,  Fig.  106, 
is  excavated  to  the  line  AB,  and  the  right- 
hand  half  of  the  roof  arch  is  built.  The 
space  above  the  arch  is  then  refilled  and  the 
surface  of  the  street  restored,  after  which 
the  left-hand  trench  is  dug  and  the  side 
wall  and  roof-arch  masonry  is  built  just  as 
in  the  opposite  half.  Generally  the  work 
is  prosecuted  by  opening  up  lengths  of 
trench  at  considerable  intervals  along  the 
street  and  alternately  on  the  left-  and  right-hand  sides.  By 
this  method  one-half  of  the  street  width  is  everywhere  open 
to  traffic,  the  travel  simply  passing  from  one  side  of  the  street 
to  the  other  to  avoid  the  excavation.  When  the  lining  has 
been  completed,  the  center  core  of  earth  inclosed  by  it  is 
removed  from  the  entrances  and  shafts,  leaving  the  tunnel 
finished  except  for  the  invert  and  track  construction,  etc. 

Transverse  Trenches.  —  The  transverse  trench  or  "  slice  " 
method  of  open-cut  tunneling  has  been  employed  in  one  work, 
the  Boston  Subway.  This  method  is  described  in  the  specifica- 
tions for  the  work  prepared  by  the  chief  engineer,  Mr.  H.  A. 
Carson,  M.  Am.  Soc.  C.  E.,  as  follows :  — 


FIG.  108.  — Side -Wall 
Foundation  Con- 
struction Open-Cut 
Tunnels. 


OPEN-CUT   TUNNELING   METHODS  183 

"Trenches  about  12  ft  wide  shall  be  excavated  across  the 
street  to  as  great  a  distance  and  depth  as  is  necessary  for  the 
construction  of  the  subway.  The  top  of  this  excavation  shall 
be  bridged  during  the  night  by  strong  beams  and  timbering, 
whose  upper  surface  is  flush  with  the  surface  of  the  street. 
These  beams  shall  be  used  to  support  the  railway  tracks  as  well 
as  the  ordinary  traffic.  In  each  trench  a  small  portion  or  slice 
of  the  subway  shall  be  constructed.  Each  slice  of  the  subway 
thus  built  is  to  be  properly  joined  in  due  time  to  the  contiguous 
slices.  The  contractor  shall  at  all  times  have  as  many  slice- 
trenches  in  process  of  excavation,  in  process  of  being  filled  with 
masonry,  and  in  process  of  being  back-filled  with  earth  above 
the  completed  masonry,  as  is  necessary  for  the  even  and  steady 
progress  of  the  work  towards  completion  at  the  time  named  in 
the  contract." 

In  regard  to  the  success  of  this  method  Mr.  Carson,  in  his 
fourth  annual  report  on  the  Boston  Subway  work,  says : 

"  The  method  was  such  that  the  street  railway  tracks  were 
not  disturbed  at  all,  and  the  whole  surface  of  the  street,  if  de- 
sired, was  left  in  daytime  wholly  free  for  the  normal  traffic." 

Tunnels  on  the  Surface.  —  It  occasionally  happens  when 
filling-in  is  to  take  place  in  the  future,  or  where  landslides 
are  liable  to  bury  the  tracks,  that  a  railway  tunnel  has  to  be 
built  on  the  surface  of  the  ground.  In  such  cases  the  construc- 
tion of  the  tunnel  consists  simply  in  building  the  lining  of  the 
section  on  the  ground  surface  with  just  enough  excavation  to 
secure  the  proper  grade  and  foundation.  Generally  the  lining 
is  finished  on  the  outside  with  a  waterproof  coating,  and  is 
sometimes  banked  and  partly  covered  with  earth  to  protect  the 
masonry  from  falling  stones  and  similar  shocks  from  other 
causes.  A  recent  example  of  tunnel  construction  of  this  char- 
acter was  described  in  "  Engineering  News  "  of  Sept.  8,  1898. 
In  constructing  the  Golden  Circle  Railroad,  in  the  Cripple  Creek 
mining  district  of  Colorado,  the  line  had  to  be  carried  across  a 
valley  used  as  a  dumping-ground  for  the  refuse  of  the  surround- 


184  TUNNELING 

ing  mines.  To  protect  the  line  from  this  refuse,  the  engineer 
constructed  a  tunnel  lining  consisting  of  successive  steel  ribs, 
filled  between  with  masonry. 

Concluding  Remarks.  —  From  the  fact  that  the  open-cut 
method  of  tunneling  consists  first  in  excavating  a  cut,  and  sec- 
ond in  covering  this  cut  to  form  an  underground  passageway, 
it  has  been  named  the  l*  cut-and-cover "  method  of  tunneling. 
The  cut-and-cover  method  of  tunneling  is  almost  never  employed 
elsewhere  than  in  cities,  or  where  the  surface  of  the  ground  has 
to  be  restored  for  the  accommodation  of  traffic  and  business. 
When  it  is  not  necessary  to  restore  the  original  surface,  as  is 
usually  the  case  with  tunnels  built  in  the  ordinary  course  of 
railway  work,  it  would  obviously  be  absurd  to  do  so  except  in 
extraordinary  cases.  In  a  general  way,  therefore,  it  may  be  said 
that  the  cut-and-cover  method  of  construction  is  confined  to  the 
building  of  tunnels  under  city  streets ;  and  the  discussion  of 
this  kind  of  tunnels  follows  logically  the  general  description  of 
the  open-cut  method  of  tunneling  which  has  been  given. 

TUNNELS  UNDER  CITY  STREETS. 

The  three  most  common  purposes  of  tunnels  under  city 
streets  are :  to  provide  for  the  removal  of  railway  tracks  from 
the  street  surface,  and  separate  the  street  railway  traffic  from 
the  vehicular  and  pedestrian  traffic;  to  provide  for  rapid 
transit  railways  from  the  business  section  to  the  outlying 
residence  districts  of  the  city ;  and  to  provide  conduits  for  sew- 
age or  subways  for  water  and  gas  mains,  sewers,  wires,  etc. 
Within  recent  years  the  greatest  works  of  tunneling  under  city 
streets  have  been  designed  and  carried  out  to  furnish  improved 
transit  facilities. 

Condition^  of  Work.  —  The  construction  of  tunnels  under  city 
streets  may  be  divided  into  two  classes,  which  may  be  briefly 
defined  as  shallow  tunnels  and  deep  tunnels.  Shallow  tunnels, 
or  those  constructed  at  a  small  depth  beneath  the  surface,  are 


OPEN-CUT    TUNNELING    METHODS  185 

usually  built  by  one  of  the  cut-and-cover  methods ;  deep 
tunnels,  or  those  built  at  a  great  depth,  beneath  the  surface 
are  constructed  by  any  of  the  various  methods  of  tunneling 
described  in  this  book,  the  choice  of  the  method  depending 
upon  the  character  of  the  material  penetrated,  and  the  local 
conditions. 

In  building  tunnels  under  city  streets  the  first  duty  of  the 
engineer  is  to  disturb  as  little  as  possible  the  various  existing 
structures,  and  the  activities  for  which  these  structures  and  the 
street  are  designed.  The  character  of  the  difficulties  encoun- 
tered in  performing  this  duty  will  depend  upon  the  depth  at 
which  the  tunnel  is  driven.  In  constructing  shallow  tunnels 
by  the  cut-and-cover  method  care  has  to  be  taken  first  of  all 
not  to  disturb  the  street  traffic  any  more  than  is  absolutely 
necessary.  This  condition  precludes  the  single  trench  method 
of  open  cut  tunneling  in  all  places  where  the  street  traffic  is  at 
all  dense,  and  compels  the  engineer  to  use  the  parallel  trench 
method  employed  in  Paris,  as  previously  described,  or  else  the 
transverse  trench  or  slice  method  employed  in  the  Boston 
Subway. 

Both  of  these  methods  have  to  be  modified  when  the  work 
is  done  on  streets  having  underground  trolley  and  cable  roads, 
and  in  which  are  located  gas  and  water  pipes,  conduits  for 
wires,  etc.  Where  underground  trolley  or  cable  railways  are 
encountered,  a  common  mode  of  procedure  is  to  excavate 
parallel  side  trenches  for  the  side  walls,  and  turn  the  roof  arch 
until  it  reaches  the  conduit  carrying  the  cables  or  wires.  The 
earth  is  then  removed  from  beneath  the  conduit  structure  in 
small  sections,  and  the  arch  completed  as  each  section  is 
opened.  As  fast  as  the  arch  is  completed  the  conduit  struc- 
ture is  supported  on  it.  Where  pipes  are  encountered  they 
may  be  supported  by  means  of  chains,  suspending  them  from 
heavy  cross-beams,  or  by  means  of  strutting,  or  they  may  be 
removed  and  rebuilt  at  a  new  level.  Generally  the  conditions 
require  a  different  solution  of  this  problem  at  different  points. 


186  TUNNELING 

Another  serious  difficulty  of  tunneling  under  city  streets 
arises  from  the  danger  of  disturbing  the  foundations  of  the 
adjacent  buildings.  This  danger  exists  only  where  the  depth 
of  the  tunnel  excavation  extends  below  the  depth  of  the  build- 
ing foundations,  and  where  the  material  penetrated  is  soft 
ground.  Where  the  tunnel  penetrates  rock  there  is  no  danger 
of  disturbing  the  building  foundations.  To  prevent  trouble  of 
this  character  requires  simply  that  the  excavation  of  the 
tunnel  be  so  conducted  that  there  is  no  inflow  of  the  surround- 
ing material,  which  may,  by  causing  a  settlement  of  the  neigh- 
boring material,  allow  the  foundations  resting  on  it  to  sink. 

The  Baltimore  Belt  tunnel,  described  in  a  succeeding  chap- 
ter, is  an  example  of  the  method  of  work  adopted  in  construct- 
ing a  tunnel  under  city  streets  through  very  soft  ground. 
This  may  be  classed  as  a  deep  tunnel.  Another  method  of 
deep  tunneling  under  city  streets  is  the  shield  method,  ex- 
amples of  which  are  given  in  a  preceding  chapter.  Two 
notable  examples  of  cut-and-cover  methods  of  tunneling  are 
the  Boston  Subway  and  the  New  York  Rapid  Transit  Ry.,  a 
description  of  which  follows. 

Boston  Subway.  —  The  Boston  Subway  may  be  defined  as  the 
underground  terminal  system  of  the  surface  street  railway 
system  of  the  city,  and  as  such  it  comprises  various  branches, 
loops,  and  stations.  The  subway  begins  at  the  Public  Garden 
on  Boylston  St.,  near  Charles  St.,  and  passes  with  double 
tracks  under  Boylston  St.  to  its  intersection  with  Tremont  St., 
where  it  meets  the  other  double-track  branch,  passing  under 
Tremont  St.  and  beginning  at  its  intersection  with  Shawmut 
Ave.  From  their  intersection  at  Tremont  and  Boylston  streets 
the  two  double-track  branches  proceed  under  Tremont  St.  with 
four  tracks  to  Scollay  Square.  At  Scollay  Square  the  subway 
divides  again  into  two  double-track  branches,  one  passing 
under  Hanover  St.,  and  the  other  under  Washington  St.  At 
the  intersection  of  Hanover  and  Washington  streets  the  two 
double-track  branches  combine  again  into  a  four-track  line, 


OPEN-CUT   TUNNELING    METHODS 


1ST 


which  runs  under  Washington  St.  to  its  terminus  at  Hay- 
market  Square,  where  it  comes  to  the  surface  by  means  of  an 
incline.  The  subway,  therefore,  has  three  portals  or  entrances, 
located  respectively  at  Boylston  St.,  Shawmut  Ave.,  and  Hay- 
market  Square.  It  also  has  five  stations  and  two  loops,  the 
former  being  located  at  Boylston  St.,  Park  St.,  Scollay  Square, 
Adams  Square,  and  Haymarket  Square,  and  the  latter  at  Park 
St.  and  Adams  Square.  The  total  length  of  the  subway  is 
10,810  ft. 

Material  Penetrated.  —  The  material  met  with  in  construct- 
ing the  subway  is  alluvial  in  character,  the  lower  strata  being 
generally  composed  of  blue  clay  and  sand,  and  the  upper  strata 
of  more  loose  soil,  such  as  loam,  oyster  shells,  gravel,  and  peat. 
At  many  points  the  material  was  so  stable  that  the  walls  of 
the  excavation  would  stand  vertical  for  some  time  after  excava- 
,tion.  Surface  water  was  encountered,  but  generally  in  small 
quantities,  except  near  the  Boylston  St.  portal,  where  it  was 
so  plentiful  as  to  cause  some  trouble. 

Cross- Section.  —  The  subway  being  built  for  two  tracks  in 
some  places  and  for  four  tracks  in  other  places,  it  was  neces- 
sary to  vary  the  form  and  dimensions  of  the  cross-section. 
The  cross-sections  actually 
adopted  are  of  three  types. 
Fig.  109  shows  the  section 
known  as  the  wide  arch  type, 
in  which  the  lining  is  solid 
masonry.  The  second  type 
was  known  as  the  double- 
barrel  section,  and  is  shown 
by  Fig.  110.  The  third  type 
of  section  is  shown  by  Fig.  111.  The  lining  consists  of  steel 
columns  carrying  transverse  roof  girders ;  the  roof  girders 
being  filled  between  with  arches,  and  the  wall  columns  having 
concrete  walls  between  them.  The  wide-arch  type  and  the 
double-barrel  type  of  sections  were  employed  in  some  portions 


FIG.  109.— Wide  Arch  Section,  Boston  Subway. 


188 


TUNNELING 


of  the  Tremont  St.  line,  where  the  traffic  was  very  dense, 
since  it  was  possible  to  construct  them  without  opening  the 
street.  Much  of  the  wide  arch  line  was  constructed  by  the 
use  of  the  roof  shield,  which  is  described  in  the  succeeding 
chapter  on  the  shield  system  of  tunneling. 

Methods  of  Construction.  —  Several  different  methods  were 
employed  in  constructing  the  subway.  Where  ample  space 
was  available,  the  single  wide  trench  method  of  cut-and-cover 


FIG.  110.  — Double  Barrel  Section,  Boston  Subway. 

construction  was  employed,  the  earth  being  removed  as  fast  as 
•excavated.  In  the  streets,  except  where  regular  tunneling  was 
resorted  to,  the  parallel  trench  or  transverse  trench  cut-and- 
-cover  methods  were  employed. 

In  the  transverse  trench  method,  trenches  about  12  ft.  wide 
were  excavated  across  the  street,  their  length  being  equal  to 
the  extreme  transverse  width  of  the  tunnel  lining,  and  their 
depth  being  equal  to  the  depth  of  the  tunnel  floor.  These 
trenches  were  begun  during  the  night,  and  immediately  roofed 


OPEN-CUT    TUNNELING    METHODS 


189 


over  with  a  timber  platform  flush  with  the  street  surface. 
Under  these  platforms  the  excavation  was  completed  and  the 
lining  built.  As  each  trench  or  "  slice "  was  completed,  the 
street  above  it  was  restored  and  the  platform  reconstructed 


Cross  Section  of  Side  Wall. 


Tile   '  W-proofingA 

HAYMARKET    SQUARE 


Cross  Section  of  Roof  . 


FIG.  111.  —  Four  Track  Rectangular  Section,  Boston  Subway. 

over  the  succeeding  trench  or  slice.  During  the  construction 
of  each  slice  the  street  traffic,  including  the  street  cars,  was 
carried  by  the  timber  platform. 

In  the  parallel  trench  method,  short  parallel  trenches  were 
dug  for  the  opposite  side  walls,  and  also  for  the  intermediate 


Waterproofing  • 
FIG.  112.  — Section  Showing  Slice  Method  of  Construction,  Boston  Subway. 

columns,  and  completely  roofed  over  during  the  night.  Under 
this  roofing  the  masonry  of  the  side  walls  and  column  founda- 
tions and  the  columns  themselves  were  erected.  When  the 
side  walls  and  columns  had  been  erected,  the  surface  of  the 
street  between  them  was  removed,  the  roof  beams  laid,  and  a 


190  TUNNELING 

platform  covering  erected,  as  shown  by  Fig.  112.  This  roofing 
work  was  also  done  at  night.  The  subsequent  work  of  build- 
ing the  roof  arches,  removing  the  remainder  of  the  earth,  and 
constructing  the  invert,  was  carried  on  underneath  the  plat- 
form covering  which  carried  the  street  traffic  in  the  meantime. 
The  successive  repetition  of  the  processes  described  con- 
structed the  subway. 

Where  the  traffic  was  very  dense  on  the  street  above,  tunnel- 
ing was  resorted  to.  For  small  portions  of  this  work  the  ex- 
cavation was  done  in  the  ordinary  way,  using  timber  strutting, 
but  much  the  greater  portion  of  the  tunnel  work  was  performed 
by  means  of  a  roof  shield.  In  the  latter  case,  the  side  walls 
were  first  built  in  small  bottom  side  drifts  and  were  fitted  with 
tracks  on  top  to  carry  the  roof  shield.  The  construction  and 
operation  of  this  shield  are  described  fully  in  the  succeeding 
chapter  on  the  shield  system  of  tunneling. 

Masonry.  —  The  masonry  of  the  inclined  approaches  to  the 
subway  consists  simply  of  two  parallel  stone  masonry  retaining 
walls.  In  the  wide-arch  and  double-barrel  tunnel  sections,  the 
side  walls  are  of  concrete  and  the  roof  arches  are  of  brick  masonry. 
In  the  other  parts  of  the  subway  the  masonry  consists  of  brick 
jack  arches  sprung  between  the  roof  beams  and  covered  with 
concrete,  of  concrete  walls  embedding  the  side  columns,  and 
of  the  concrete  invert  and  foundations  for  the  columns.  Figs. 
109  to  112  inclusive  show  the  general  details  of  the  masonry 
work  for  each  of  the  three  sections.  The  inside  of  the  lining 
masonry  is  painted  throughout  with  white  paint. 

Stations. —  The  design  and  construction  of  the  stations  for 
the  Boston  Subway  were  made  the  subjects  of  considerable 
thought.  All  the  stations  consist  of  two  island  platforms  of 
artificial  stone  having  stairways  leading  to  the  street  above. 
The  platforms  are  made  1  ft.  higher  than  the  rails.  The  station 
structure  itself  is  built  of  steel  columns  and  roof  beams  with 
brick  roof  arches,  and  concrete  side  walls.  Its  interior  is  lined 
with  white  enameled  tiles.  The  intermediate  columns  are  cased 


OPEN-CUT   TUNNELING   METHODS  191 

with  wood,  and  have  circular  wooden  seats  at  their  bottoms. 
Each  stairway  is  covered  by  a  light  housing,  consisting  of  a 
steel  framework  with  a  copper  covering  and  an  interior  wood 
and  tile  finish. 

Ventilation.  —  The  subway  is  ventilated  by  means  of  ex- 
haust fans  located  in  seven  fan  chambers,  some  of  which  con- 
tain two  fans,  and  others  only  one  fan.  Each  of  the  fans  has  a 
capacity  of  from  30,000  to  37,000  cu.  ft.  of  air  per  minute,  and 
is  driven  by  electric  motor,  taking  current  from  the  trolley 
wires.  This  system  of  ventilation  has  worked  satisfactorily. 

Disposal  of  Rainwater.  —  The  rainwater  which  enters  the 
subway  from  the  inclined  entrances,  together  with  that  from 
leakage,  is  lifted  from  12  ft.  to  18  ft.  by  automatic  electric 
pumps  to  the  city  sewers.  The  subway  has  pump- wells  at  the 
Public  Garden,  at  Eliot  St.,  Adams  Square,  and  Haymarket 
Square.  In  each  of  these  wells  are  two  vertical  submerged 
centrifugal  pumps  made  entirely  of  composition  metal.  In 
each  chamber  above,  are  two  electric  motors  operating  the 
pumps.  Each  motor  is  started  and  stopped  according  to  the 
height  of  water  by  means  of  a  float  and  an  automatic  release 
starting  box.  The  floats  are  so  placed  that  only  one  pump 
is  usually  brought  into  use.  The  other,  however,  comes  into 
service  in  case  the  first  pump  is  out  of  order  or  the  water 
enters  more  rapidly  than  one  pump  can  dispose  of  it.  In  the 
latter  case,  both  motors  continue  to  run  until  the  same  low 
level  has  been  reached. 

Very  little  dampness  except  from  atmospheric  condensation 
is  to  be  found  on  tlie  interior  walls  or  roof  of  the  subway, 
although  numerous  discolored  patches,  caused  by  dampness  and 
dust,  may  be  seen  on  some  parts  of  the  walls.  Substantially  all 
of  the  leakage  comes  through  the  small  drains  in  the  invert 
leading  from  hollows  left  in  the  side  walls.  Careful  measure- 
ment was  taken  at  the  end  of  an  unusually  wet  season  to  de- 
termine the  actual  amount  of  leakage,  and  the  total  amount  for 
the  entire  subway  was  found  to  be  about  81  gallons  per  minute. 


192  TUNNELING 

Estimated  Quantities.  —  The  estimated  quantities  of  material 
used  in  constructing  the  subway  were  as  follows : 

Excavation 369,450  cu.  yds. 

Concrete 75,660  "     " 

Brick 11,105  "     " 

Steel 8,105  tons 

Granite 2,285  cu.  yds. 

Piles 117,925  lin.  ft. 

Ribbed  tiles 12,440  sq.  yds. 

Plaster 88,190  " 

Waterproofing  (asphalt  coating)  .     .     .  117,980  " 

Artificial  stone 6,790  " 

Enameled  brick 2,210  " 

Enameled  tiles 2,855  •' 

Cost  of  the  Subway.  —  The  estimated  cost  of  the  subway  made 
before  the  work  was  begun  was  approximately  14,000,000,  and 
the  cost  of  construction  did  not  exceed  13,700,000.  This 
includes  ventilating  and  pump  chambers,  changes  of  water  and 
gas  pipes,  sewers  and  other  structures,  administration,  engineer- 
ing, interest  on  bonds,  and  all  cost  whatsoever.  Dividing  this 
number  by  the  total  length  we  obtain  a  cost  per  linear  foot  of 
1342.30. 

New  York  Rapid  Transit  Railway The  project  of  an  under- 
ground rapid  transit  railway  to  run  the  entire  length  of  Man- 
hattan Island,  was  originated  some  years  previous  to  1890.  In 
1894,  however,  a  Rapid  Transit  Commission  was  appointed  to 
prepare  plans  for  such  a  road,  and  after  a  large  amount  of 
trouble  and  delay  this  commission  awarded  the  contract  for 
construction  to  Mr.  John  B.  McDonald  of  New  York  City,  on 
Jan.  15,  1900.  Not  enough  work  has  been  done  to  enable  a 
description  of  the  methods  of  construction,  but  the  following  is 
a  brief  account  of  the  work  to  be  done : 

Route.  —  The  road  starts  from  a  loop  which  encircles  the 
triangular  area  occupied  by  the  City  Hall  Park  and  the  Post- 
Office.  Within  this  loop  the  tunnel  construction  will  be  two- 
storied;  but  where  the  main  line  leaves  the  loop,  all  four  tracks 


OPEN-CUT    TUNNELING    METHODS  193 

will  come  to  the  same  level,  and  continue  side  by  side  thereafter 
except  at  the  points  which  will  be  noted  as  the  description 
proceeds.  Proceeding  from  the  loop,  the  four-track  line  passes 
under  Center  and  Elm  Streets.  It  will  continue  under  Lafay- 
ette Place,  across  Astor  Place  and  private  property  between 
As  tor  Place  and  Ninth  St.  to  Fourth  Ave.  The  road  will  then 
pass  under  Fourth  and  Park  avenues  until  42d  St.  is  reached. 
At  this  point  the  line  turns  west  along  42d  St.,  which  it 
follows  to  Broadway.  It  turns  northward  again  under  Broad- 
way to  the  boulevard,  crossing  the  Circle  at  59th  St.  The  road 
will  then  follow  the  boulevard  until  97th  St.  is  reached,  where 
the  four-track  line  is  separated  into  two  double-track  lines. 

At  a  suitable  point  north  of  96th  St.  the  outside  tracks  will 
rise  so  as  to  permit  the  inside  tracks,  on  reaching  a  point  near 
103d  St.,  to  curve  to  the  right,  passing  under  the  north-bound 
track,  and  to  continue  thence  across  and  under  private  property 
to  104th  St.  From  there  the  two-track  tunnel  will  go  under 
104th  St  and  Central  Park  to  110th  St.,  near  Lenox  Ave. ; 
thence  under  Lenox  Ave.  to  a  point  near  142d  St. ;  thence 
across  and  under  private  property  and  the  intervening  streets 
to  the  Harlem  River.  The  road  will  pass  under  the  Harlem 
River  and  across  and  under  private  property  to  149th  St., 
which  street  it  will  follow  to  Third  Ave.,  and  will  then  pass 
under  Westchester  Ave.,  where  at  a  convenient  point  the  tracks 
will  emerge  from  the  tunnel,  and  be  carried  on  a  viaduct  along 
and  over  Westchester  Ave.,  Southern  Boulevard,  and  Boston 
Road  to  Bronx  Park.  This  portion  of  the  line,  from  96th  St. 
to  Bronx  Park,  will  be  known  as  the  East  Side  Line. 

From  the  northern  side  of  96th  St.  the  outside  tracks  will 
rise,  and  after  crossing  over  the  inside  tracks  they  will  be 
brought  together  on  a  location  under  the  center  line  of  the 
street  and  proceed  along  under  the  boulevard  to  a  point  between 
122d  and  123d  streets.  At  this  point  the  tracks  will  com- 
mence to  emerge  from  the  tunnel,  and  be  carried  on  a  viaduct 
along  and  over  the  boulevard  at  a  point  between  134th  and 


194  TUNNELING 

135th  streets,  where  they  will  again  pass  into  the  tunnel  under 
and  along  the  boulevard  and  Eleventh  Ave.  to  a  point  about 
1,350  ft.  north  of  the  center  line  of  190th  St.  There  the  tracks 
will  again  emerge  from  the  tunnel,  and  be  carried  on  a  viaduct 
across  and  over  private  property  to  El  wood  St.,  and  over  and 
along  Elwood  St.  to  Kingsbridge  St.  to  Kingsbridge  Ave., 
private  property,  the  Harlem  Ship  Canal  and  Spuyten  Duyvil 
Creek,  private  property,  Riverdale  Ave.,  or  230th  St.  to  a  ter- 
minus near  Bailey  Ave.  That  portion  of  the  line  from  96th 
St.  to  the  above  mentioned  terminus  at  Bailey  Ave.  will  be 
known  as  the  West  Side  Line. 

The  total  length  of  "the  rapid  transit  road,  including  the 
parts  above  and  below  the  surface  ground  of  the  streets,  as  well 
as  both  the  East  and  West  Side  Lines,  will  be  about  20^  miles. 

Material  Penetrated.  —  The  soil  through  which  the  road  will 
be  excavated,  according  to  numerous  borings  taken  along  the 
line,  will  be  a  varied  one.  The  lower  portion  of  the  road,  or 
the  part  including  the  loop  around  the  Post-Office  up  to  nearly 
Fourth  St.,  will  be  undoubtedly  excavated  through  loose  soil, 
but  from  Fourth  St.  to  the  ends  it  will  be  excavated  in  rock. 
The  loose  soil  forming  the  southern  part  of  Manhattan  Island 
is  chiefly  composed  of  clay,  sand,  and  old  rubbish  —  a  soil  very 
easy  to  excavate.  There  is  no  fear  of  any  damage  to  the  build- 
ings along  the  line  since,  with  the  exception  of  the  loop  around 
the  Post-Office,  no  high  buildings  are  met ;  and  at  the  loop  the 
underground  road  passes  far  above  the  plane  of  the  foundations 
of  the  high  buildings  fronting  Park  Row.  Water  will  be  met 
at  some  points,  but  not  in  such  quantities  as  to  be  a  serious 
inconvenience,  except,  perhaps,  in  crossing  Canal  St.,  where  the 
meeting  of  a  large  body  of  water  is  expected.  From  Fourth  St. 
to  the  ends  of  both  the  east  and  west  side  lines,  the  soil  will  be 
chiefly  composed.of  rock  of  gneissoid  and  mica-schistose  char- 
acter, these  rocks  prevailing  nearly  throughout  the  whole  of 
Manhattan  Island.  The  rock,  as  a  rule,  will  not  be  compact, 
but  will  have  seams  and  fissures,  and  at  many  points  it  will  be 


OPEN-CUT    TUNNELING   METHODS 


195 


found  disintegrated  and  alternated  with  strata  of  loose  soils, 
and  even  pockets  of  quicksand  will  be  met  with  along  the  line 
of  the  road. 

Cross-Sections.  —  The  section  of  the  underground  road  will 
be  of  three  different  types, —  the  rectangular,  the  barrel- vault, 
and  the  circular.  The  rectangular  section,  Fig.  113,  will  be 
used  for  the  greater  part  of  the  road,  of  which  a  portion  will 
be  for  four  tracks  and  a  portion  for  two  tracks.  The  dimensions 
adopted  for  the  four  tracks  are  50  x  13  ft.,  and  for  the  double 
tracks  25  x  13  ft.  The  barrel-vault  section,  composed  of  a 


Water Proofing       Minimum  Tfiidnea  lobe  8: 

tntckntss  Increased  in  Beta  unx/ftd. 

FIG.  113.  — Double  Track  Section,  New  York  Rapid  Transit  Railway. 

polycentric  arch,  having  the  flattest  curve  at  the  crown,  whose 
dimensions  are  16  ft.  high  and  24  ft.  wide,  has  been  adopted 
for  the  portions  of  the  road  to  be  tunneled.  The  circular  sec- 
tion of  15-ft.  diameter  will  be  used  under  the  Harlem  River, 
and  being  for  single  track,  two  parallel  tunnels  will  be  built 
side  by  side. 

The  main  line  from  the  post-office  loop  to  about  102d 
St.,  consists  of  four  tracks  built  side  by  side  in  one  conduit, 
except  for  that  portion  under  the  present  Fourth  Ave. 
tunnel  where  two  parallel  double-track  tunnels  will  be  em- 
ployed. The  West  Side  Line  will  consist  of  double  tracks 


196  TUNNELING 

laid  in  one  conduit,  except  across  Manhattan  St.  and  beyond 
190th  St.,  where  it  will  be  carried  on  an  elevated  structure. 
The  East  Side  Line  will  consist  of  a  double-track  tunnel  driven 
from  102d  St.,  and  the  boulevard  under  Central  Park  to 
110th  St.  and  Lenox  Ave.,  and  two  parallel  circular  tun- 
nels excavated  under  the  Harlem  River,  —  the  other  portions 
of  the  road  being  double-track,  subway  and  elevated  structure. 
The  subway,  both  for  four  and  two  tracks,  may  be  built  by 
open  excavation,  cut-and-cover  methods. 

For  the  main  line  the  Slice  method,  so  successfully  em- 
ployed in  the  Boston  Subway,  will  be  adopted  as  the  most 
convenient  in  a  case  where  the  width  of  the  excavation  is 
great  and  the  traffic  enormous,  as  is  the  case  especially  below 
43d  St.  and  along  the  boulevard.  For  the  double-track  sub- 
way, the  method  of  the  side  trench  will  perhaps  be  adopted 
on  account  of  it  being  the  least  expensive ;  and  since  the 
streets  where  such  a  trench  will  be  opened  are  very  wide,  with 
only  a  light  traffic. 

Lining.  —  The  lining  of  the  subway  is  of  concrete,  carried 
by  a  framework  of  steel.  The  floor  consists  of  a  foundation 
layer  of  concrete  at  least  eight  inches  thick  on  good  founda- 
tion, but  thicker,  according  to  conditions,  where  the  founda- 
tion is  bad.  On  top  of  this  is  placed  another  layer  of  concrete, 
with  a  layer  of  waterproofing  between  the  two.  In  this  top 
layer  are  set  the  stone  pedestals  for  the  steel  columns,  and  the 
members  making  up  the  tracks. 

In  the  four-track  subway,  the  steel  framework  consists  of 
transverse  bents  of  columns,  and  I-beams  spaced  about  five  feet 
apart  along  the  tunnel.  The  three  interior  columns  of  each 
bent  are  built  up  bulb  angle  and  plate  columns  of  H-section. 
The  wall  columns  are  I-beams,  as  are  also  the  roof  beams ; 
between  the  I-beams,  wall  columns,  and  roof  beams  there  is  a 
concrete  filling.  So  that  the  roof  of  the  subway  will  be  made 
up  of  concrete  arches  resting  on  the  flanges  of  the  I-beams  of 
the  roof.  The  concrete  to  be  used  is  of  one  part  Portland 


OPEN-CUT    TUNNELING    METHODS 


197 


cement,  two  parts  sand,  and  four  parts  broken  stones.  The 
double-track  subway  will  be  built  in  the  same  way,  except  that 
only  one  column  is  placed  between  the  tracks  for  the"  support 
of  the  roof. 

All  the  concrete  masonry  of  the  roof,  foundations,  and  side 
walls,  must  contain  a  layer  of  waterproofing,  so  as  to  keep 
perfectly  dry  the  underground  road,  and  prevent  the  perco- 
lation of  water.  This  waterproofing  must  be  made  up  as 
follows :  On  the  lowest  stratum  of  concrete,  whose  surface 
is  made  as  smooth  as  possible,  a  layer  of  hot  asphalt  is  spread. 
On  this  asphalt  are  immediately  laid  sheets  or  rolls  of  felt ; 
another  layer  of  hot  asphalt  is  then  spread  over  the  felt,  and 


tTtf  -— Jftf- — 


FIG.  114.  —Park  Avenue  Deep  Tunnel  Construction,  New  York  Rapid  Transit  Railway. 

then  another  layer  of  felt  laid,  and  so  on,  until  no  less  than 
two,  and  no  more  than  six,  layers  of  felt  are  laid,  with  the  felt 
between  layers  of  asphalt.  On  top  of  the  upper  surface  of 
asphalt  the  remainder  of  the  concrete  is  put  in  place  so  as  to 
reach  the  required  thickness  of  the  concrete  wall. 

Tunnels.  —  At  three  points  the  Standard  Subway  will  be 
replaced  by  tunnel  lines.  The  location  of  the  three  tunnels 
will  be  between  33d  and  42d  St.  on  Park  Ave. ;  under 
Central  Park,  northeast  of  104th  St.,  and  under  the  Har- 
lem River.  The  Park  Ave.  construction  (Fig.  114)  will 
consist  of  two  parallel  double-track  tunnels,  located  on  each 
side  of  the  street,  and  about  10  ft.  below  the  present  tunnel. 
The  soil  being  composed  of  good  rock,  the  tunnels  will  be 


198 


TUNNELING 


driven  by  a  wide  heading,  and  one  bench,  since  no  strutting 
will  be  required,  and  the  masonry  lining,  even  of  the  roof,  may 
be  left  far  behind  the  front  of  the  excavation.  The  masonry 
lining  will  consist  of  concrete  walls  and  brick  arches.  The 
tunnel  under  Central  Park  being  driven  through  a  similar 
rock,  the  same  method  of  excavation  and  the  same  manner  of 
lining  will  be  used. 

The  tunnel  under  the  Harlem  River  is  to  be  driven  through 
soft  ground ;  and  it  will  be  constructed  as  a  submarine  tunnel, 
according  to  the  shield  and  compressed  air  combined  process. 


FIG.  115.  — Harlem  River  Tunnel,  New  York  Rapid  Transit  Railway. 

The  tunnels  will  be  lined  with  iron  made  up  of  segments,  with 
radial  and  circumferential  flanges.  Concrete  will  be  placed 
inside  and  flush  with  the  flanges. 

The  tracks,  both  in  the  subway  and  tunnels,  are  an  inti- 
mate part  of  the  concrete  flooring.  The  rail  rests  on  a  con- 
tinuous bearing  of  wooden  blocks,  laid  with  the  grain  running 
transversely  with  respect  to  the  line  of  the  rail,  and  held  in 
place  by  two  channel  iron  guard  rails.  The  guard  rails  are 
bolted  to  metal  cross-ties  embedded  in  the  concrete. 


OPEN-CUT    TUNNELING   METHODS  199 

Viaduct.  —  A  considerable  portion  of  the  double  track  branch 
lines  north  of  103d  St.  will  be  viaduct,  or  elevated  structure. 
The  viaduct  construction  on  the  West  Side  Line  will  extend,  in- 
cluding approaches,  from  122d  St.  to  very  near  135th  St. 
Of  this  distance,  2,018  ft.  8  ins.  will  be  viaduct  proper, 
consisting  of  plate  girder  spans  carried  by  trestle  bents  at  the 
ends,  and  by  trestle  towers  for  the  central  portion.  The 
columns  of  the  bents  and  towers  are  to  be  built  up  bulb-angle 
and  plate  columns  of  H-section  of  the  same  form  as  those  used 
in  the  bents  inside  the  subway.  The  approaches  proper  will  be 
built  of  masonry.  The  elevated  line  proper  consists  of  plate 
girder  spans,  supported  on  plate  girder  plate  cross  girders 
carried  by  columns  set  at  the  curb  lines. 

Stations.  —  Many  stations  will  be  built  along  the  line. 
These  will  be  located  on  each  side  of  the  street.  The 
entrances  at  the  stations  will  consist  of  iron  framework,  with 
glass  roofs  covering  the  descending  stairways.  The  passage- 
ways leading  down  will  be  walled  with  white  enameled  bricks 
and  wainscoted  with  slabs  of  marble.  The  stations  for  the 
local  trains  will  be  located  on  each  side  of  the  road  close  to  the 
walls,  since  the  outside  tracks  are  reserved  for  the  local  trains, 
while  the  middle  ones  will  be  reserved  for  the  expresses.  The 
few  stations  for  the  express  trains  will  be  located  between  the 
middle  and  outside  tracks.  Stations  will  be  provided  with  all 
the  conveniences  required,  having  toilet  rooms,  news  stands, 
benches,  etc.,  and  will  be  lighted  day  and  night  by  numerous 
arc  lamps. 

G-eneral.  —  The  contractor  is  compelled  to  complete  the 
work  in  four  and  one-half  years,  but  he  has  promised  to  have 
it  in  full  running  order  within  three  years.  There  is  no  diffi- 
culty in  doing  this,  since  the  great  extension  of  the  road  and 
the  great  width  of  the  avenues  under  which  it  runs  allow  work 
all  along  the  line  at  the  same  time.  The  work,  briefly  summar- 
ized, comprises  the  following  items  :  — 


200  TUNNELING 

Length  of  all  sections,  ft 109,570 

Total  excavation  of  earth,  cu.  yds 1,700,228 

Earth  to  be  filled  back,            " .     .  773,093 

Rock  excavated,                       " 921,128 

Rock  tunneled,                          " 368,606 

Steel  used  in  structure,  tons 65,044 

Cast  iron  used,                   "...<. 7,901 

Concrete,  cu.  yds 489,122 

Brick,             "          ..............  18,519 

Waterproofing,  sq.  yds 775,795 

Vault  lights,            "      .',';.'.     .     . 6,640 

Local  stations,  number      ......' 43 

Express  stations,  "           5 

Station  elevators,  "           10 

Track  total,  lin.  ft 305,380 

"      underground,  lin.  ft 245,514 

"      elevated,             "        59,766 

In  addition  to  the  construction  of  the  railway  itself,  it  will 
be  necessary  to  construct  or  reconstruct  certain  sewers,  and  to 
adjust,  readjust,  and  maintain  street  railway  lines,  water  pipes, 
subways,  and  other  surface  and  subsurface  structures,  and  to 
relay  street  pavements. 

The  total  cost  of  the  work,  according  to  the  contract  signed 
by  Mr.  McDonald,  will  be  135,000,000.  Dividing  this  amount 
by  the  total  length  of  the  road,  which  is  109,570  lineal  feet, 
we  have  the  unit  cost  a  lineal  foot  $315,  or  a  little  less  than 
unit  of  cost  of  the  Boston  subway,  which  was  $342  per 
lineal  foot. 

The  road  belongs  to  the  city.  The  contractor  acts  as  an 
agent  for  the  city  in  carrying  out  the  work,  and  he  is  the  leaser 
of  the  road  for  fifty  years.  The  work  is  paid  for  as  soon  as 
the  various  parts  of  the  road  are  completed,  and  the  money  is 
obtained  from  an  issue  of  city  bonds.  During  the  fifty  years' 
lease  the  contractor  will  pay  the  interest  plus  1  %  of  the  face 
value  of  the  bonds.  This  1  %  goes  to  the  sinking-fund,  which 
within  the  fifty  years  at  compound  interest  forms  the  total  sum 
required  for  the  redemption  of  bonds. 


SUBMARINE   TUNNELING  201 


CHAPTER    XVIII. 

SUBMARINE     TUNNELING:     GENERAL     DISCUS- 
SION.—THE    SEVERN    TUNNEL. 


GENERAL    DISCUSSION. 

SUBMARINE  tunnels,  or% tunnels  excavated  under  the  beds  of 
rivers,  lakes,  etc.,  have  been  constructed  in  large  numbers 
during  the  last  quarter  of  a  century,  and  the  projects  for  such 
tunnels,  which  have  not  yet  been  carried  to  completion,  are 
still  more  numerous.  Among  the  more  notable  completed 
works  of  this  character  may  be  noted  the  tunnel  under  the 
River  Severn  and  those  under  the  River  Thames  in  England, 
the  one  under  the  River  Seine  in  France,  that  under  the 
St.  Clair  River  for  railway,  that  under  the  East  River  for  gas 
mains,  that  under  Dorchester  Bay,  Boston,  for  sewage,  and 
those  under  Lakes  Michigan  and  Erie  for  the  water  supply  of 
Chicago  and  Cleveland  in  America.  Among  the  partly  com- 
pleted submarine  tunnels  which  have  been  abandoned  the  most 
notable  example  is,  perhaps,  the  Hudson  River  tunnel.  For 
the  details  of  the  various  projected  submarine  tunnels  of  note, 
which  include  tunnels  under  the  English  and  Irish  Channels, 
under  the  Straits  of  Gibraltar,  under  the  sound  between 
Copenhagen  in  Denmark  and  Malino  in  Sweden,  under  the 
Messina  Straits  between  Italy  and  Sicily,  and  under  the  Straits 
of  Northumberland  between  New  Brunswick  and  Prince 
Edward  Island,  the  reader  is  referred  to  the  periodical  litera- 
ture of  the  last  few  years. 

Previous  to  attempting  the  driving  of  a  submarine  tunnel 
it  is  necessary  to  ascertain  the  character  of  the  material  it  will 


202  TUNNELING 

penetrate.  This  fact  is  generally  determined  by  making  dia- 
mond-drill borings  along  the  line,  and  the  object  of  ascertaining 
it  is  to  determine  the  method  of  excavation  to  be  adopted.  If 
the  material  is  permeable  and  the  tunnel  must  pass  at  a  small 
depth  below  the  river  bed,  a  method  will  have  to  be  adopted 
.which  provides  for  handling  the  difficulty  of  inflowing  water. 
If,  on  the  other  hand,  the  tunnel  passes  through  impermeable 
material  at  a  great  depth,  there  will  be  no  unusual  trouble 
from  water,  and  almost  any  of  the  ordinary  methods  of  tun- 
neling such  materials  may  be  employed.  Shallow  submarine 
tunnels  through  permeable  material  are  usually  driven  by  the 
shield  method  or  by  the  compressed  jiir  method,  or  by  a  method 
which  is  a  combination  of  the  first  and  second. 

It  is  not  an  uncommon  experience  for  a  submarine  tunnel 
to  start  out  in  firm  soil  and  unexpectedly  to  find  that  this 
material  becomes  soft  and  treacherous  as  the  wwk  proceeds,  or 
that  it  is  intersected  by  strata  of  soft  material.  The  method  of 
dealing  with  this  condition  will  vary  with  the  circumstances,  but 
generally  if  any  considerable  amount  of  soft  material  has  to  be 
penetrated,  or  if  the  inflow  of  water  is  very  large,  the  firm- 
ground  system  of  work  is  changed  to  one  of  the  methods 
employed  for  excavating  soft-ground  submarine  tunnels.  The 
Milwaukee  water  supply  tunnel  and  the  East  River  gas  tunnel, 
described  elsewhere,  are  notable  examples  of  submarine  tunnels 
began  in  firm  material  which  unexpectedly  developed  a  treacher- 
ous character  after  the  work  had  proceeded  some  distance. 
Occasionally  the  task  of  building  a  submarine  tunnel  in  the 
river  bed  arises.  In  such  cases  the  tunnel  is  usually  built  by 
means  of  cofferdams  in  shallow  water,  and  by  means  of  caissons 
in  deep  water. 

Submarine  tunnels  under  rivers  are  usually  built  with  a  de- 
scending grade  from  each  end  which  terminates  in  a  level  middle 
position,  the  longitudinal  profile  of  the  tunnel  corresponding  to 
the  transverse  profile  of  the  river  bottom.  Where,  however, 
such  tunnels  pass  under  the  water  with  one  end  submerged,  and 


SUBMARINE    TUNNELING  203 

the  other  end  rising  to  land  like  the  water  supply  tunnels  of 
Chicago,  Milwaukee,  and  Cleveland,  the  longitudinal  profile  is 
commonly  level,  or  else  descends  from  the  shore  to  a  level 
position  reaching  out  under  the  water. 

The  drainage  of  submarine  tunnels  during  construction  is 
one  of  the  most  serious  problems  with  which  the  engineer  has 
to  deal  in  such  works.  This  arises  from  the  fact  that,  since  the 
entrances  of  the  tunnel  are  higher  than  the  other  parts,  all  of 
the  seepage  water  remains  in  the  tunnel  unless  pumped  out,  and 
from  the  possibility  of  encountering  faults  or  permeable  strata, 
which  reach  to  the  stream  bed  and  give  access  to  water  in 
greater  or  less  quantities.  Generally,  therefore,  the  excavation 
is  conducted  in  such  a  manner  that  the  inflowing  water  is  led 
directly  to  sumps.  To  drain  these  sumps  pumping  stations 
are  necessary  at  the  shore  shafts,  and  they  should  have  ample 
capacity  to  handle  the  ordinary  amount  of  seepage,  and  enough 
surplus  capacity  to  meet  probable  increases  in  the  inflow.  For 
extraordinary  emergencies  this  plant  may  have  to  be  greatly 
enlarged,  but  it  is  not  usual  to  provide  for  these  at  the  outset 
unless  their  likelihood  is  obvious  from  the  start.  The  character 
and  size  of  the  pumping  plants  used  in  constructing  a  number 
of  well-known  tunnels  are  described  in  Chapter  XII. 

In  this  book  submarine  tunnels  will  be  classified  as  follows: 
(1)  Tunnels  in  rock  or  very  compact  soils,  which  are  driven  by 
any  of  the  ordinary  methods  of  tunneling  similar  materials  011 
land;  (2)  tunnels  in  loose  soils,  which  may  be  driven,  (a)  by 
compressed  air,  (5)  by  shields,  or  (c)  by  shields  and  compressed 
air  combined;  (3)  tunnels  on  the  river  bed,  which  are  con- 
structed, (a)  by  cofferdams,  or  (&)  by  caissons ;  (4)  tunnels 
partly  in  firm  soil  and  partly  in  treacherous  soils,  which  are 
driven  partly  by  one  of  the  firm-soil  methods,  and  partly  by  one 
of  the  soft-soil  methods.  To  illustrate  tunnels  of  the  first  class, 
the  River  Severn  tunnel  in  England  has  been  selected ;  to 
illustrate  those  of  the  second  class,  the  several  tunnels  discussed 
in  the  chapter  on  the  shield  system  of  tunneling  and  the  Mil- 


204  TUNNELING 

waukee  tunnel  is  sufficient ;  to  illustrate  those  of  the  third  class, 
the  Yan  Buren  Street  tunnel  in  Chicago  is  selected  ;  and  to 
illustrate  those  of  the  fourth  class,  the  East  River  gas  tunnel 
and  the  Milwaukee  water  supply  tunnels  are  excellent  examples. 

THE  SEVERN  TUNNEL. 

The  Severn  tunnel,  which  carries  the  Great  Western  Hail- 
way,  of  England,  beneath  the  estuary  01  a  large  river,  is  4  miles 
642  yards  long.  It  penetrates  strata  of  conglomerate,  limestone, 
carboniferous  beds,  marl,  gravel,  and  sand,  at  a  minimum  depth 
of  44f  ft.  below  the  deepest  portion  of  the  estuary  bed.  The 
following  description  of  the  work  is  abstracted  from  a  paper  by 
Mr.  L.  F.  Yernon-Harcourt.  * 

The  first  work  was  the  sinking  of  a  shaft,  15  ft.  in  diameter, 
lined  with  brickwork,  on  the  Monmouthshire  bank  of  the  Severn, 
200  ft.  deep.  After  the  Monmouthshire  shaft  had  been  sunk,  a 
heading  7  ft.  square  was  driven  under  the  river,  rising  with  a 
gradient  of  1  in  500  from  the  shaft  on  the  Monmouthshire  shore, 
so  as  to  drain  the  lowest  part  of  the  tunnel.  Near  to  the  first,  a 
second  shaft  was  sunk  and  tubbed  with  iron,  in  which  the 
pumps  were  placed  for  removing  the  water  from  the  tunnel 
works,  connection  being  made  by  a  cross-heading  with  the 
heading  from  the  first  shaft.  There  was  also  a  shaft  on  the 
Gloucestershire  shore ;  and  two  shafts  inland  from  the  first  on 
the  Monmouthshire  side,  to  expedite  the  construction  of  the 
tunnel.  Headings  were  then  driven  in  both  directions  along  the 
line  of  the  tunnel,  from  the  four  shafts ;  and  the  drainage  head- 
ing from  the  old  shaft  was  continued,  in  the  line  of  the  tunnel, 
under  the  deep  channel  of  the  estuary,  and  up  the  ascending 
gradient  towards  the  Gloucestershire  shore,  till,  in  October, 
1879,  it  had  reached  to  within  about  130  yards  of  the  end  of 
the  descending  heading  from  the  Gloucestershire  shaft.  During 
this  period,  though  the  work  had  progressed  slowly,  no  large 

*  Proceedings  Inst.  C.E.,  vol.  cxxi. 


SUBMARINE    TUNNELING  205 

quantity  of  water  had  been  met  with  in  any  of  the  headings,  in 
spite  of  their  already  extending  under  almost  the  whole  width 
of  the  estuary.  On  October  18,  1889,  however,  a  great  spring 
was  tapped  by  the  heading  which  was  being  driven  landwards 
from  the  old  shaft,  about  40  ft.  above  the  level  of  the  drainage 
heading ;  and  the  water  poured  out  from  this  land  spring  in 
such  quantity  that,  flowing  along  the  heading,  falling  down  the 
old  shaft,  and  thus  finding  its  way  into  the  drainage  heading 
and  the  long  heading  of  the  tunnel  under  the  estuary  in  con- 
nection with  it,  it  flooded  the  whole  of  the  workings  in  com- 
munication with  the  old  shaft,  which  it  also  tilled  within  twenty- 
four  hours  from  the  piercing  of  the  spring. 

To  obtain  increased  security  against  any  influx  of  water 
from  the  deep  channel  of  the  estuary  into  the  tunnel,  the 
proposed  level  portion  of  the  tunnel,  rather  more  than  a 
furlong  long  under  this  part,  was  lowered  15  ft.  by  increas- 
ing the  descending  gradient  on  the  Monmouthshire  side  from 
1  in  100  to  1  in  90,  and  lowering  the  proposed  rail  level  on. 
the  Gloucestershire  side  15  ft.  throughout  the  ascent,  so  as  not 
to  increase  the  gradient  of  1  in  100  against  the  load.  A 
new  shaft,  18  ft.  in  diameter,  was  sunk  slightly  nearer  the 
estuary  011  the  Monmouthshire  shore  than  the  old  one ;  two 
shafts  also  were  sunk  on  the  land  side  of  the  great  spring  for 
pumping  purposes;  and  additional  pumping  machinery  was 
erected.  The  flow  from  the  spring  into  the  old  shaft  was 
arrested  by  a  shield  of  oak  fixed  across  the  heading;  and 
at  last,  after  numerous  failures  and  breakdowns  of  the  pumps, 
the  headings  were  cleared  of  water,  after  a  diver,  supplied  with 
a  knapsack  of  compressed  oxygen,  had  closed  a  door  in  the 
long  heading  under  the  estuary ;  and  the  works  were  resumed 
nearly  fourteen  months  after  the  flooding  occurred.  The  great 
spring  was  then  shut  off  from  the  workings  by  a  wall  across 
the  heading  leading  to  the  old  shaft ;  and,  owing  to  the  lower- 
ing of  the  level  of  the  tunnel,  a  new  drainage  heading  had  to 
be  driven  from  the  bottom  of  the  new  shaft  at  a  lower  level, 


206  TUNNELING 

which  was  made  5  ft.  in  diameter,  and  lined  with  brickwork, 
whilst  the  old  drainage  heading  was  enlarged  to  9  ft.  in  diam- 
eter, and  lined  with  brickwork,  so  as  to  aid  in  the  permanent 
ventilation  of  the  tunnel.  The  lowering  of  the  level,  moreover, 
converted  the  bottom  tunnel  headings  into  top  headings,  so 
that  along  more  than  a  mile  of  the  tunnel  the  semicircular  arch, 
26  ft.  in  diameter,  was  built  first,  and  then,  after  lowering  the 
headings,  the  invert  was  laid  and  the  side  walls  were  built  up. 
Bottom  headings  were  driven  along  the  remainder  of  the  tunnel, 
and  the  work  was  expedited  by  means  of  break-ups.  Ventila- 
tion was  effected  in  the  works  by  a  fan  18  inches  in  diameter 
and  7  ft.  wide,  fixed  at  the  top  of  the  new  deep  shaft ;  the  rock 
was  bored  by  drills  worked  by  compressed  air ;  the  blasting  was 
eventually  effected  exclusively  by  tonite,  owing  to  its  being 
freer  from  deleterious  fumes  than  any  other  explosive  ;  and  the 
workings  were  lighted  by  Swan  and  Brush  electric  lamps.  The 
tunnel  is  lined  throughout  with  vitrified  brickwork,  between 
2]  ft.  to  3  ft.  thick,  set  in  cement,  and  has  an  invert  1|  ft.  to 
3  ft.  in  thickness ;  the  lining  was  commenced  towards  the  end 
of  1880,  the  headings  under  the  river  were  joined  in  Septem- 
ber, 1881,  and  the  last  length  of  the  tunnel,  across  the  line  of 
the  great  spring,  was  completed  in  April,  1885. 

Water  came  in  from  the  river  through  a  hole  in  a  pool  of 
the  estuary,  close  to  the  Gloucestershire  shore,  in  April,  1881, 
during  the  lining  of  a  portion  of  the  tunnel,  but  fortunately 
before  the  headings  were  joined.  This  influx  was  stopped  by 
allowing  the  water  to  rise  in  the  tunnel  to  tide-level,  to  prevent 
the  enlargement  of  the  hole,  which  was  then  filled  up  at- low 
water  with  clay,  weighted  on  the  top  with  clay  in  bags.  The 
great  spring  broke  out  again  in  October,  1883,  and  flooded  the 
works  a  second  time ;  but  within  four  weeks  the  water  had 
been  pumped  out  and  the  spring  again  imprisoned.  During 
this  period  an  exceptionally  high  tide,  raised  still  higher  by 
a  southwesterly  gale,  inundated  the  low-lying  land  on  the  Mon- 
mouthshire side  of  the  estuary,  and,  flowing  down  one  of  the 


SUBMARINE    TUNNELING  207 

inland  shafts,  flooded  a  section  of  the  tunnel,  but  the  pumps 
removed  this  water  within  a  week. 

In  order  to  construct  the  portion  of  tunnel  traversing  the 
line  of  the  great  spring,  the  water  was  diverted  into  a  side 
heading  below  the  level  of  the  tunnel,  leading  to  the  old  shaft, 
whence  it  was  pumped,  and  the  fissure  below  the  tunnel  was 
filled  with  concrete,  over  which  the  invert  was  built.  An 
attempt  to  imprison  the  spring,  on  the  completion  of  this 
length  of  tunnel,  having  resulted  in  imposing  an  excessive  pres- 
sure on  the  brickwork,  leading  to  fractures  and  leakage,  a  shaft, 
29  ft.  in  diameter,  was  sunk  at  the  side  of  the  tunnel  at  this 
point  in  1886,  and  pumps  were  erected  powerful  enough  to 
deal  with  the  entire  flow  of  the  spring. 

The  tunnel  was  opened  for  traffic  in  December,  1886,  and 
gives  access  to  a  double  line  of  railway,  connecting  the  lines 
converging  to  Bristol  with  the  South  Wales  railway  and  the 
western  lines.  The  pumping  power  provided  at  the  shaft  con- 
nected with  the  great  spring,  and  at  four  other  shafts,  is  capable 
of  raising  66,000,000  gallons  of  water  per  day,  the  maximum 
amount  pumped  from  the  tunnel  being  30,000,000  gallons  a 
day.  The  ventilation  of  the  tunnel  is  effected  by  fans  placed 
in  the  two  main  shafts  on  each  bank  of  the  estuary,  and  the  fan 
in  the  Monmouthshire  shaft  is  40  ft.  in  diameter,  and  12  ft. 
wide.  The  tunnel  gives  passage  to  a  large  traffic,  numerous 
through-trains  between  the  north  and  southwest  of  England 
making  use  of  it. 


208  TUNNELING 


CHAPTER   XIX. 

SUBMARINE  TUNNELING  (Continued) ;    THE  EAST 

RIVER  GAS  TUNNEL.  — VAN  BUREN  ST. 

TUNNEL,  CHICAGO. 


THE  East  River  gas  tunnel  is  a  notable  example  of  a  tunnel 
begun  in  firm  soil  which  unexpectedly  developed  treacherous 
strata.  It  is  also  remarkable  from  the  fact  that  the  shield  which 
was  employed  to  overcome  the  trouble  was  driven  from  rock 
into  soft  material  and  from  the  soft  material  into  rock  again 
with  the  utmost  success.  The  following  description  of  the 
work  is  abstracted  from  a  paper  by  Mr.  Walton  I.  Aims,  the 
engineer  in  charge  of  the  work,  published  in  the  Journal  of 
the  Association  of  Engineering  Societies  for  May,  1895,  and  in 
Engineering  Neivs  of  July  11,  1895.  The  accompanying  cuts 
are  reproduced  from  the  last-named  publication. 

During  1891  and  1892  the  East  River  Gas  Co.,  of  Long 
Island  City,  a  corporation  with  works  situated  on  the  Long 
Island  shore  of  the  East  River,  obtained  from  the  New  York 
State  Legislature  a  new  charter,  and  such  necessary  legislation 
as  to  permit  the  extension  of  their  mains  across  the  East  River 
into  the  city  of  New  York. 

The  feasibility  of  constructing  a  tunnel  under  the  river 
through  which  the  gas  mains  might  be  laid  was  discussed,  and 
after  some  preliminary  surveys  and  examinations  a  route  was 
decided  upon  from  the  works  of  the  company  at  Ravenswood, 
Long  Island  City,  to  between  70th  and  71st  Sts.,  New  York, 
passing  under  Blackwell's  Island  and  the  east  and  west  chan- 
nels of  the  East  River.  On  about  this  line  of  location  some 
eight  or  ten  pipe  soundings  were  made  in  the  two  river  chan- 


SUBMARINE    TUNNELING  209 

nels,  all  of  which  indicated  a  rock  bottom ;  and  the  results  of 
these,  together  with  surface  indications,  where  at  both  the  Long 
Island  and  New  York  shores,  as  well  as  on  Blackwell's  Island, 
bedrock  lay  exposed,  led  all  to  conclude  that  nothing  but  rock 
was  to  be  encountered.  On  these  investigations  a  contract  was 
entered  into  on  June  25,  1891,  for  the  construction  of  a  sup- 
posedly rock  tunnel,  which  the  contractor  guaranteed  to  com- 
plete by  April,  1893. 

Work  was  begun  at  the  Ravenswood  or  Long  Island  side 
on  June  28  by  sinking  a  shaft  9  ft.  square  about  200  ft.  back 
from  the  river  to  a  depth  of  about  148  ft.  below  the  surface ; 
while  at  New  York,  on  July  7,  a  shaft  of  the  same  dimensions 
was  sunk  to  a  depth  from  the  surface  of  139  ft.  In  both  these 
shafts  rock  was  entered  after  about  8  ft.  of  soil ;  but  while  the 
rock  at  New  York  was  quite  dry,  at  Ravenswood  it  proved 
seamy  and  very  wet. 

The  tunnel-roof  grade  had  been  established  at  109  ft.  below 
mean  high  water  at  the  New  York  shaft,  with  a  grade  for  drain- 
age of  ^  °/0  towards  Ravenswood.  This  gave  a  minimum  cover 
of  41  ft.  at  the  deepest  point  in  the  west  or  New  York  channel 
on  the  East  River,  where  there  is  70  ft.  of  water  at  mean  high 
tide.  The  east  or  Long  Island  channel  is  comparatively  shallow, 
the  deepest  point  being  only  35  ft.  below  mean  high  water  level. 
The  one  thing  feared  was  that  fissures  yielding  large  volumes 
of  water  might  extend  to  the  tunnel  roof  and  largely  augment 
the  cost  of  pumping.  The  size  of  the  tunnel  section  was  to  be 
8  ft.  6  ins.  in  height  by  10  ft.  6  ins.  in  width,  this  giving  suffi- 
cient room  for  the  laying  of  two  3-ft.  gas  mains  and  one  4-ft. 
main. 

In  the  shafts,  on  both  sides  of  the  river,  the  headings  were 
now  turned.  At  Ravenswood  the  work  was  delayed  by  meet- 
ing considerable  quantities  of  silty  water,  but  at  New  York  the 
tunnel  was  practically  dry  until  towards  the  end  of  December, 
1892,  when,  at  a  distance  of  338  ft.  from  the  shaft,  a  fissure 
was  struck  yielding  about  a  3-in.  stream  of  salt  water.  The 


210  TUNNELING 

rock  to  within  20  ft.  of  this  point  had  been  the  regular  hard 
New  York  gneiss,  with  a  dip  towards  Long  Island  of  10°  from 
the  vertical,  and  a  strike  north  and  south  at  right  angles  to  the 
direction  of  the  tunnel.  Here  it  gradually  began  to  soften, 
becoming  more  and  more  micaceous  until  when  about  20  ft. 
beyond  the  water-bearing  fissure  the  rock  suddenly  terminated, 
running  into  a  vein  of  soft  material  with  the  same  dip  and 
strike  as  that  of  the  rock. 

This  new  material  proved  to  be  a  vein,  principally  of  decom- 
posed feldspar,  gray  in  color,  crumbling  easily,  and  with  no 
perceptible  grit.  It  still  preserved  a  rock  structure,  and  was 
perfectly  dry  when  undisturbed.  But  its  exposed  surfaces 
were  quickly  acted  upon  by  water,  which  it  would  absorb  and 
then  wash  away  quite  rapidly.  The  water-bearing  fissure  and 
this  soft  vein  were  connected ;  more  water  was  also  met  at  the 
junction  of  the  rock  and  the  soft  material,  and  later  experience 
proved  that  in  passing  through  these  soft  veins  water  was 
always  to  be  found  next  to  the  rock  —  a  sort  of  water-course 
on  both  sides  of  the  soft  vein.  Had  it  not  been  for  encounter- 
ing this  water,  the  tunnel  might  have  been  carried  through  the 
soft  material  without  employing  compressed  air,  though  the 
prudence  of  attempting  this  might  be  questioned,  for  nothing 
more  insures  the  safety  of  both  the  men  and  the  work  than 
compressed  air  in  sub-aqueous  tunneling. 

The  finding  of  this  soft  material,  so  unexpected,  was  quite 
a  set-back  to  all  concerned.  However,  it  was  decided  to  drive 
a  small  timbered  drift  about  4  ft.  wide  by  6  ft.  high  to  investi- 
gate the  ground  ahead,  and  find  how  much  of  this  material  was 
to  be  penetrated  before  solid  rock  was  again  met.  This  drift 
was  started  and  driven  in  for  about  6  ft.  Meanwhile  a  most 
destructive  action  was  going  on  between  the  water  and  the  soft 
material.  The  water  running  along  the  face  of  the  rock  had 
washed  out  a  cavity  overhead  in  the  soft  ground.  The  walls 
of  this  cavity  were  gradually  breaking  away,  and  the  clay-like 
substance  falling  down  would  close  the  outlet  of  the  water  into 


SUBMARINE   TUNNELING  211 

the  tunnel.  The  water  would  then  accumulate  in  this  pocket, 
softening  up  fresh  material  on  the  sides  until  it  had  gained  a 
sufficient  head  to  burst  through  the  dam  which  confined  it, 
when  it  would  come  rushing  into  the  tunnel,  carrying  with  it 
large  quantities  of  the  softened  material.  These  rushes  were 
accompanied  by  a  loud  bubbling  sound  that  quite  mystified  the 
men,  which  was,  of  course,  the  sound  of  the  air  displacing  the 
water  in  the  cavity.  As  soon  as  the  pocket  had  emptied  itself, 
for  a  time  the  trouble  was  over,  until  with  the  falling  of  more 
material  the  outlet  was  again  closed  and  the  operation  was 
repeated.  These  rushes  of  water,  with  the  accompanying 
sound  of  the  bubbling  air,  soon  became  more  and  more  alarming 
to  the  men.  The  cavity  was  constantly  increasing  in  size,  and 
extending  up  toward  the  river-bed.  Each  recurrence  would 
now  send  the  men  running  for  the  shaft,  by  no  means  certain 
that  the  river  had  not  at  last  made  a  connection  wiflh  the 
tunnel. 

All  work  in  the  small  drift  was  abandoned,  and  on  Dec.  31 
a  bulkhead  was  hurriedly  constructed  at  the  face  to  prevent  the 
threatened  flooding  of  the  shaft.  Up  to  this  time  over  25  yards 
of  material  had  been  washed  into  the  tunnel,  all  of  which  had 
come  from  along  the  rock  face.  With  the  river-bed  only  45  ft. 
above  the  tunnel-roof,  there  is  every  reason  to  believe  that  this 
.bulkhead  was  put  in  none  too  soon,  and  a  connection  with  the 
river  narrowly  averted.  The  bulkhead  was  well  packed  with 
hay  to  prevent,  as  much  as  possible,  further  washing  of  the 
material,  and  a  discussion  was  now  entered  into  as  to  the 
method  of  future  procedure.  The  contractors  were  in  favor  of 
abandoning  the  heading  and  returning  to  the  shaft,  to  sink  to  a 
lower  level  and  start  anew  in  hopes  of  meeting  more  favorable 
conditions  at  a  greater  depth.  There  had  been  a  somewhat 
similar  experience  on  the  Croton  Aqueduct,  where  that  tunnel 
passes  under  the  Harlem  River.  Soft  material  had  been  en- 
countered on  the  first  established  level,  which  proved  so  trouble- 
some that  after  two  or  three  unsuccessful  attempts  had  been 


212  TUNNELING 

made  to  pass  through  it,  it  was  finally  decided  to  abandon  the 
heading  and  return  to  the  shaft,  sinking  some  150  ft.  deeper. 
On  this  new  level  nothing  but  rock  was  encountered.  In  the 
East  River  tunnel,  however,  the  soft  material  was  clearly  a 
decomposed  vein,  and  to  what  depth  this  decomposition  might 
extend  was  unknown ;  so  that  as  there  were  no  well-founded 
reasons,  in  this  case,  for  expecting  any  better  conditions  at  a 
lower  level,  it  was  decided  to  first  attempt  to  drive  the  present 
heading,  in  compressed  air,  leaving  the  sinking  as  a  later  ex- 
pedient should  the  proposed  means  fail.  An  arrangement  was 
made  with  the  contractors  by  which  the  company  was  to  share 
the  expense  of  the  work  in  soft  ground. 

It  was  at  this  time  that  the  writer  became  connected  with, 
the  work,  having  charge  of  installing  and  conducting  the  com- 
pressed air  operations  for  the  company.  To  form  the  com- 
pressed air-working  chamber,  a  solid  brick  wall  or  bulkhead 
8  ft.  thick  was  built  across  the  tunnel  into  gains  -in  the  rock 
about  40  ft.  back  from  the  heading,  and  containing  a  cylindrical 
steel  air-lock  6  ft.  in  diameter  and  10  ft.  long. 

In  the  engine  room,  the  18  x  24-in.  Ingersoll  piston-inlet 
compressor,  used  heretofore  for  running  the  rock-drills,  was 
supplemented  by  a  small  Rand  compressor,  and  both  arranged 
to  supply,  independently,  compressed  air  to  the  working  cham- 
ber below.  Incandescent  electric  lighting  was  introduced  into 
the  tunnel,  which  is  almost  a  necessity  in  compressed  air  opera- 
tions, as  common  illuminants  produce  an  enormous  quantity  of 
smoke  when  burning  in  compressed  air.  A  telephone  was  also 
taken  into  the  working  chamber,  by  which  instant  communica- 
tion could  be  had  with  the  engine  room  in  case  any  sudden 
increase  of  air  pressure  should  be  desired. 

On  Feb.  25,  1893,  operations  were  commenced,  in  the 
heading,  under  35  Ibs.  of  air  pressure.  The  previous  work 
here  had  greatly  increased  the  difficulties,  and  it  was  not  long 
before  the  air  pressure  had  to  be  raised  to  42  Ibs.  to  control 
the  water.  The  excavation  was  advanced  under  a  cylindrical 


SUBMARINE   TUNNELING  213 

steel  roof,  built  up  of  plates  3  ft  long  and  1  ft.  wide,  of  $-in. 
sheet  steel,  to  the  four  sides  of  which  were  riveted  angle  bars 
2i  X  2|  x  £  in.  These  plates  were  bolted  together  in  a  heading 
about  6  ft.  high.  In  the  erection  of  this  roof,  poling-boards 
were  used  for  each  plate,  and  a  bulkhead  carried  down  with 
each  ring  as  erected.  When  the  heading  had  been  advanced 
about  20  ft.  from  the  rock,  a  12  x  12  in.  yellow-pine  mudsill 
was  introduced  along  the  bottom  of  the  heading,  and  on  this 
the  roof  was  covered  by  means  of  radial  timber  bracing.  The 
excavation  was  now  carried  down  on  both  sides  of  this  mudsill, 
to  a  distance  of  about  10  ft.  from  the  rock,  the  steel  roof  being 
extended  well  down  on  the  sides.  A  circular  section  was  thus 
excavated,  in  which  brickwork  was  laid,  four  courses  thick,  and 
with  an  internal  diameter  of  10ft.  Between  March  4  and  6 
a  great  deal  of  trouble  was  experienced.  Air  pressure  was 
several  times  to  48  Ibs.,  and  the  work  progressed  very  slowly 
on  account  of  the  many  inrushes  of  water  and  softened  mate- 
rial. It  was  not  until  April.  8  that  the  last  section  of  brickwork 
in  the  soft  material  was  completed  and  rock  again  entered,  after 
passing  through  29  ft.  of  this  decomposed  material.  Of  the 
material  met  in  driving  through  this  vein,  at  first  9  ft.  of  the 
gray  decomposed  feldspar  was  penetrated,  a  vein  of  4  ins.  of 
hard  quartz  was  then  met,  and  this  was  followed  by  6  ft.  of  pure 
white  decomposed  feldspar,  smooth  and  soft  as  plaster.  The 
remaining  14  ft.  was  made  up  of  layers  of  feldspar  and  chlorite. 
This  chlorite,  deep  green  in  color,  flaky  and  grease-like  to  the 
touch  when  wet,  proved  to  be  very  troublesome  material,  as  it 
was  easily  converted  into  a  fluid  state  by  the  water,  which  was 
again  encountered  next  to  the  rock. 

At  the  Long  Island  shaft,  the  work  up  to  this  time  had  pro- 
gressed to  about  250  ft.  from  the  shaft.  The  material  so  far 
encountered  on  this  side  was  a  hard,  seamy  gneiss,  bearing  con- 
siderable quantities  of  salty  water,  containing  iron,  lime,  and 
magnesia.  Soft  ground  was  now  met  at  this  end,  in  a  seam 
about  4  ft.  wide,  of  chlorite.  As  this  material  was  perfectly 


214  TUNNELING 

dry  and  not  thoroughly  disintegrated,  the  tunnel  was  timbered 
through  this  seam  without  difficulty.  Several  similar  veins 
were  thus  met  and  passed  through,  until  at  a  point  285  ft.  from 
the  shaft,  where  after  drilling  for  about  2  ft.  through  rock  a  soft 
green,  almost  liquid  chlorite  vein  was  struck,  which  began  flowing 
in  through  the  drill  holes  with  great  force.  These  holes  were 
plugged ;  but  as  it  was  necessary  to  know  what  was  ahead,  and 
as  with  100  ft.  of  cover  between  the  tunnel  roof  and  the  river 
bottom  it  was  thought  that  the  condition  of  affairs  could  not  be 
very  serious,  it  was  decided  to  continue  driving  ahead  without 
air  pressure,  and  with  a  timbered  heading.  To  see  what  the 
material  would  do,  several  hand-holes  were  put  into  the  rock- 
face  with  the  object  of  blasting  out  a  hole  about  2  ft.  square 
through  the  remaining  2  ft.  of  rock,  to  the  chlorite.  Before 
blasting,  however,  the  precaution  was  taken  to  build  a  bulk- 
head, some  40  ft.  back  from  the  face.  On  firing  the  holes  an 
inrush  of  many  yards  of  material  took  place,  which  was  finally 
checked  by  some  rock  fragments  closing  the  opening  through 
the  rock.  After  several  desperate  attempts  on  the  part  of  the 
contractors  to  control  this  material  and  make  progress,  the  work 
was  finally  abandoned  in  the  latter  part  of  March,  and  as  a 
4-in.  stream  of  water  was  now  flowing  from  the  heading,  pump- 
ing was  discontinued,  and  the  shaft  and  tunnel  allowed  to  flood. 
At  the  New  York  end  work  was  still  being  carried  on  in 
compressed  air.  The  rock  encountered  at  the  other  side  of  the 
soft  seam  closely  resembled  the  decomposed  material  which  had 
been  penetrated  before,  and  consisted  of  alternate  layers  of 
feldspar  and  chlorite,  with  an  occasional  vein  of  quartz.  It 
was  quite  soft,  though  requiring  drilling  and  blasting,  and 
eventually  it  had  to  be  lined.  After  the  heading  had  been 
driven  about  69  ft.  into  this  rock  the  company  decided,  in 
spite  of  the  uncertainty  as  to  the  material  ahead,  to  remove  the 
air  pressure,  and  to  call  upon  the  contractors  to  resume  their 
contract.  Upon  removing  the  air  pressure,  however,  the  brick- 
work through  the  soft  seam  proved  so  unsatisfactory  in  exclud- 


SUBMARINE   TUNNELING 


215 


flrt 


ing  the  water  that  air  pressure  was  again  put  on,  and  it  was 
decided  to  line  the  brickwork  with  a  circular  cast-iron  lining 
(Fig.  116).  Although  this  brickwork  was  only  10  ft.  in  in- 
side diameter,  a  lining  was  designed  10  ft.  2  ins.  in  the  clear, 
as  it  was  now  desired  to  make  the  tunnel  bore  as  large  as 
possible.  To  put  in  this  lining,  some  of  the  brickwork  had  to 
be  cut  out,  which  was  then  removed  in  sections,  enough  for 
one  ring  of  plates  at  a  time.  The  lining  consisted  of  rings 

of  plates  or  segments, 

mo***±—*n  ru.iTt"! 

1  ft.  4  ins.  wide,  with 
internal  flanges  4  ins. 
deep,  from  the  back  of 
the  plate.  The  metal 
in  both  the  back  of 
the  plate  and  the 
flanges  was  1^  ins. 
thick.  All  the  joint- 
faces  of  the  segments 
were  planed,  and  1-in. 
bolts  used  for  fasten- 
ing them  together.  A 
complete  tunnel  ring 
was  composed  of  nine  segments  and  a  small  inverted  key, 
about  8  ins.  wide. 

Difficulties  between  the  company  and  the  contractors,  which 
had  been  brewing  for  some  time,  now  culminated  and  the 
courts  were  appealed  to,  to  settle  their  differences.  This 
caused  a  cessation  of  work  for  a  short  time  until  the  com- 
pany were  empowered  to  take  possession  and  resume  the  work 
of  construction  for  themselves.  The  work  of  putting  the  cast- 
iron  lining  into  the  brickwork  was  necessarily  a  very  slow 
operation.  The  lining  was  extended  well  into  the  rock  on 
both  sides  of  the  soft  vein,  and  a  wall  built  at  both  ends,  be- 


Ooss  Section. 

Long:  Section. 

FIG.  116.  —  Sections  of  Cast  Iron  Lining,  East  River 
Gas  Tunnel. 


216  TUNNELING 

tween  the  rock  and  the  iron  lining,  to  confine  the  Portland 
cement  grout,  which  was  now  introduced  back  of  the  plates. 
To  effect  this  grouting  1^-in.  holes  had  been  drilled  and  tapped 
through  the  back  of  several  plates  in  each  ring.  Through 
these  holes  the  grout  was  pumped  by  means  of  a  Cameron 
pump ;  and  after  the  space  between  the  brickwork  and  the 
lining  had  been  thoroughly  grouted,  the  work  was  found,  on 
taking  off  the  air  pressure  from  the  heading,  to  be  perfectly 
water-tight.  It  was  not  until  towards  the  end  of  July  that  the 
work  of  lining  the  brickwork  was  completed  and  driving  ahead 
in  the  rock  was  resumed.  Then,  when  an  advance  of  only  10 
ft.  had  been  made,  a  second  soft  seam  was  encountered  about 
80  ft.  beyond  the  first  one,  and  a  test  pipe  was  driven  to  a 
horizontal  depth  of  70  ft.,  without  encountering  anything 
solid.  To  avoid  further  delay,  the  driving  of  the  test-pipe 
was  discontinued  at  this  depth,  and  preparations  made  for 
advancing  the  heading.  For  this  test- pipe  1^-in.  common 
wrought-iron  pipe  was  used,  which  was  driven  in  by  a  small 
machine-drill,  and  washed  out  at  each  lengthening  of  the 
pipe  with  a  l|-in.  wash-pipe.  From  these  washings  the  differ- 
ent materials  penetrated  were  sampled,  with  the  following 
tabulated  results  : 

3  ft.  gray  decomposed  feldspar  and  chlorite. 

11  ft.  soft  black  mud,  containing  lumps  of  carbonized  wood  like  charcoal. 
19  ft.  hard  black  mud  and  sand,  with  nodules  of  pyrites. 
22  ft.  gray  decomposed  feldspar. 

4  ft.  decomposed  feldspar  and  chlorite. 
11  ft.  gray  decomposed  feldspar. 

Water  was  again  found  next  to  the  rock,  but  was  consider- 
ably held  in  check  by  the  compressed  air.  As  from  the  results 
of  the  test-pipe  there  were  no  special  difficulties  to  apprehend 
from  the  indicated  material,  it  was  decided  to  drive  ahead, 
under  the  open  heading  method,  as  this  involved  no  delays  in 
waiting  for  special  machinery.  The  light  steel  cylindrical  roof 
was  again  used  in  advancing  the  excavation,  but  for  the  perma- 


SUBMARINE   TUNNELING  217 

nent  lining  the  cast-iron  rings  were  to  be  introduced  instead 
of  brickwork,  as  heretofore.  A  start  was  made  on  Aug.  7  to 
drive  the  heading  into  the  soft  material,  but  two  days  later,  after 
the  work  had  been  advanced  6  ft.  into  the  soft  vein,  orders 
were  received  to  suspend  all  work  on  account  of  the  great 
financial  depression  of  the  time.  This  was  unfortunate ;  and 
could  it  have  been  anticipated  a  few  days  the  heading  into  the 
soft  material  would  have  been  left  unopened.  As  it  was  now, 
from  being  first  disturbed  and  then  abandoned,  the  water  was 
first  allowed  to  soften  up  the  black  mud  in  the  heading,  and,  in 
spite  of  the  bulkhead,  a  considerable  quantity  of  the  material 
was  washed  into  the  tunnel. 

This  stay  of  proceedings  was  utilized  by  making  a  horizon- 
tal test  boring  in  the  heading  on  the  Long  Island  side.  At 
this  shaft  no  work  had  been  done  since  the  departure  of  the 
contractors,  beyond  the  building  of  a  brick  bulkhead  and  air- 
lock in  the  tunnel.  Compressed  air  had  then  been  put  on, 
which  considerably  reduced  the  amount  of  water  flowing  into 
the  tunnel  from  the  heading.  The  action  of  the  compressed 
air  had  been  somewhat  peculiar ;  for  notwithstanding  the  great 
depth  of  the  tunnel  below  the  river  bed,  at  10  Ibs.  pressure  the 
air  began  to  escape  through  the  heading,  and  with  a  pressure 
of  35  Ibs.  per  sq.  in.  small  bubbles  of  escaping  air  could  be 
seen  rising  to  the  surface  for  over  300  ft.  up  and  down  the 
river.  This  seemed  to  indicate  that  the  ground  above  the 
tunnel  had  been  honeycombed  up  to  the  river  bottom  by 
the  previous  washing-in  of  such  quantities  of  the  soft  green 
chlorite.  As  it  was  known  that  there  were  detached  lumps  of 
rocks  in  this  soft  vein,  2-in.  heavy  pipe  was  used  for  the  test 
boring,  with  drive-well  couplings,  and  a  circular,  hollow  steel 
bit  for  the  cutting  end.  This  pipe  was  driven  in  the  same  way 
as  the  one  on  the  New  York  side,  and  after  passing  through 
chlorite  and  various  kinds  of  sofkrock  fragments,  solid  rock 
was  again  met  at  32  ft.  Into  this  rock  a  hole  was  drilled  to  a 
depth  of  54  ft.,  using  a  small  bit  on  the  end  of  a  1-in.  pipe  and 


218 


TUNNELING 


drilling    through    the    test-pipe.      The    rock   beyond    the    soft 
seam  was  a  soft  white  limestone. 

With  the  prospect  of  resuming  work  the  question  now 
arose  as  to  the  best  method  of  proceeding;  and,  as  a  great 
deal  depended  upon  the  success  of  driving  through  the  present 
headings,  it  was  strongly  recommended  that  the  safest  and 
surest  method,  that  of  shield  tunneling,  be  adopted  in  both 
headings,  although  necessarily  entailing  a  large  expenditure  in 
plant,  and  delay  in  time  for  installation.  This  plan  met  with 


*&£}* 


longitudinal  Section. 


End  Viewof  Head. 


FIG.  117.  — Section  and  Elevation  of  Shield,  East  River  Gas  Tunnel. 

the  company's  approval,  and  a  shield  and  hydraulic  plant  were 
designed.  As  the  nature  of  the  material  to  be  penetrated  be- 
yond the  test-pipes  was  unknown,  this  shield  was  so  made  that 
in  passing  from  rock  to  soft  material,  or  back  again  to  rock,  it 
could  be  erected  or  taken  apart  again  with  a  minimum  of  time 
and  labor,  so  that  it  might  almost  be  called  a  portable  shield 
(Fig.  117).  As  in  both  the  tunnel  headings  there  was  but  one 
air-lock,  and  as  it  was  inadvisable  to  remove  the  air  pressure 
from  the  headings,  the  different  parts  of  the  shield  had  to  be  of 
such  size  as  could  be  passed  through  the  air-lock  doors.  This 


SUBMARINE    TUNNELING 


219 


was  accomplished  by  dividing  the  shield  transversely,  separatr 
ing  the  tail-end  section,  or  that  which  overlaps  the  tunnel,  from 
the  cutting-edge  section  containing  the  working  chambers. 
These  two  sections  were,  of  course,  circular,  11  ft.  f  in.  out- 
side diameter.  The  tail  end  section  was  3  ft.  6  ins.  long,  and 
the  cutting-edge  section  3  ft.  8  ins.  long.  Both  of  these 
sections  were  again  divided,  longitudinally,  into  four  quadrants. 
The  outside  shell,  in  both  tail-end  and  cutting-edge  sections, 
was  made  up  of  one  ^  in.  and  one  |  in.  steel  plates  riveted 
together ;  and  at  the  four  quadrant  joints,  there  were  |-in.  butt- 
straps  12  ins.  wide  running  the  whole 
length  of  the  shield  and  uniting  the 
quadrants  and  the  two  sections.  The 
middle  diaphragm,  separating  the 
cutting-edge  and  tail-end  sections, 
was  made  of  two  plates,  one  riveted 
to  each  of  the  two  sections,  and  these 
two  plates  bolted  together  with  the 
butt-straps  united  the  sections.  The 
cutting-edge  section  contained  two 
platforms,  one  vertical  and  one  hori- 
zontal, of  the  same  length  as  the 
section. 

To  erect  this  shield  the  only  rivet- 
ing necessary  was  at  the  four  butt- 
strap  joints  in  the  tail-end  section,  where  it  was  necessary  ta 
preserve  a  flush  surface  on  both  sides  of  the  outer  shell.  In 
the  cutting-edge  part  countersunk  bolts  were  used  through  the 
butt-straps.  About  380  f-in.  bolts  and  160  rivets  were  used  ta 
erect  the  shield.  Two  doors  closing  each  of  the  four  working 
chambers  were  hung  on  the  vertical  platform,  and  were  pro- 
vided with  fastenings  so  that  the  whole  face  could  be  easily 
closed. 

To   drive   the   shield  12  5-in.  hydraulic   jacks  were  used, 
designed  for  a  working  pressure  of  5,000  Ibs.  per  sq.  in.,  or 


FIG.  118.  — Elevation  and  Section 
of  Hydraulic  Jack,  East  River 
Gas  Tunnel. 


220  TUNNELING 

700  tons  on  the  whole  shield  (Fig.  118).  These  jacks  were 
controlled  by  two  block-valves,  one  placed  on  each  side  of 
the  shield.  Each  of  these  block-valves  consisted  of  six  inde- 
pendent valves  all  in  one  compact  casting,  each  of  which  had  a 
pressure  and  exhaust  stem.  Half-inch  XX  pipe  was  used  for 
connecting  each  jack  with  its  valve,  and  1-in.  hydraulic  pipe 
was  used  for  the  pressure  main,  which  was  connected  with  the 
shield  block-valves  by  three  swivel-joint  connections.  To  fur- 
nish the  pressure,  a  very  compact  little  pump,  designed  by 
Watson  &  Stillnian,  of  New  York,  was  used  without  an  accu- 
mulator, the  pressure  being  very  nicely  governed  by  a  steam- 
regulating  valve. 

On  Sept.  22  work  was  resumed  on  the  New  York  side, 
with  a  small  force  of  men  working  days  only,  to  excavate  in 
the  rock  an  enlarged  chamber  about  15  ft.  back  from  the  face, 
in  which  to  erect  the  shield.  This  chamber  was  made  circular, 
about  15  ft.  in  diameter  and  10  ft.  long.  Back  from  this,  the 
rock  was  taken  out  in  a  circular  form  of  about  11  ft.  diameter, 
for  some  14  ft.,  or  enough  for  about  10  rings  of  the  cast-iron 
segments  which  were  here  erected  in  the  rock,  the  spaces  be 
tween  being  thoroughly  grouted  with  Portland  cement.  These 
rings  were  thus  made  solid  in  the  rock  to  withstand  the  thrust 
of  the  shield-jacks  upon  the  lining.  The  blasting  necessary  in 
this  work  was  made  as  light  as  possible  ;  but  it  was  not  without 
its  effect  upon  the  soft  material  in  the  heading,  a  considerable 
quantity  of  the  black  mud  being  washed  through  the  bulkhead, 
while  the  braces  showed  signs  of  a  heavy  strain  from  the 
squeezing  of  the  material.  The  shield  arrived  at  the  works  on 
Nov.  10,  and  the  work  of  erection  was  immediately  begun. 
The  sections  were  lowered  down  the  shaft  and  taken  through 
the  air-lock  to  the  shield-chamber.  On  Nov.  17  the  shield  was 
all  assembled,  and  riveting  the  tail-end  sections  was  commenced. 
For  heating  the  rivets  in  the  air-chamber  a  forge  was  used, 
with  a  hood  to  which  was  connected  at  the  top  a  2-in.  pipe  with 
•a  valve  which  extended  through  the  air-lock  bulkhead.  By 


SUBMARINE   TUNNELING  221 

means  of  this  pipe  all  the  obnoxious  gases  from  the  furnace 
were  removed  from  the  air-chamber.  After  the  riveting  was 
finished,  the  shield  was  brought  to  its  right  position  for  line 
and  grade,  the  hydraulic  jacks  and  valves  put  in  place,  and  the 
necessary  connections  made.  On  Nov.  24  word  was  received 
that  the  work  on  the  New  York  side  was  to  be  pushed  wiih  all 
possible  speed,  and  a  force  was  at  once  organized  of  three 
gangs,  working  in  eightrhour  shifts.  More  rings  were  built  on 
the  ten  rings  already  anchored  in  the  rock,  until  the  tunnel 
lining  was  brought  within  the  tail-end  of  the  shield. 

The  shield  was  now  advanced  until  it  was  necessary  to 
disturb  the  bulkhead,  the  remaining  bench  ahead  of  the  shield 
being  blasted  out  as  the  shield  progressed.  The  most  difficult 
part  of  the  work  was  now  reached,  for  at  the  point  where  the 
shield  entered  the  soft,  black  mud  on  top  there  still  remained 
about  12  ft.  of  hard  rock  in  the  bottom,  as  the  dip  of  this  vein 
was  over  40°  toward  Long  Island.  Blasting  had  therefore  ta 
be  continued  in  the  bottom  pockets  of  the  shield  after  the  top 
had  entered  the  much-softened  material.  As  soon  as  the  bulk- 
head was  passed  it  was  with  great  difficulty  that  the  bottom 
pockets  could  be  kept  clear  of  the  black  slush  from  overhead. 
The  material  had  become  so  softened  along  the  rock  face  that 
it  was  almost  impossible  to  confine  it,  and  several  rushes  of 
inflowing  material  occurred,  until  finally  an  open  connection 
with  the  river  was  established,  and  the  tunnel  was  visited  by 
crabs  and  mussels,  together  with  boulders,  old  boots  and  shoes, 
brick,  and  tinware,  direct  from  the  river  bottom.  Notwith- 
standing these  adverse  circumstances  the  work  was  still  pro- 
gressing, although  in  45  Ibs.  of  compressed  air,  which  was  now 
escaping  through  the  heading,  and  causing  a  very  violent 
ebullition  on  the  river  surface.  This  upward  current  of  air 
held  in  check  the  downward  current  of  water,  so  that  no  efforts 
were  made  to  prevent  its  escape.  On  Dec.  13  the  shield  finally 
cleared  the  rock  and  was  now  fully  entered  into  the  soft,  black 
mud.  The  main  difficulty  was  now  surmounted,  the  work 


222  TUNNELING 

progressed  more  rapidly,  and  the  shield  soon  reached  undis- 
turbed material,  which  was  found  quite  dry  and  hard.  It  was 
still  the  same  black  mud,  with  occasional  lumps  like  chare oal, 
and  numerous  nodules  like  pyrites,  which  glistened  like  silver 
in  the  black,  peat-like  mud.  Mattocks  were  used  by  the  men 
in  the  working  chambers,  who  would  clean  out  these  four  com- 
partments to  within  a  foot  of  the  cutting  edge.  As  soon  as 
this  was  done  hydraulic  pressure  was  put  upon  the  jacks,  some- 
times to  the  amount  of  5,000  Ibs.  per  sq.  in.,  and  the  shield 
forced  ahead  16  or  18  ins.,  enough  for  another  ring  of  plates, 
the  working  chambers  again  being  filled  with  the  displaced 
material.  On  Dec.  24  the  last  of  the  black  mud  was  passed 
through,  and  lying  next  to  it,  at  an  angle  of  40°  towards 
Long  Island,  white  decomposed  feldspar  was  found,  containing 
fragments  of  decomposed  quartz  charged  with  sulphureted 
hydrogen. 

An  important  departure  was  now  made  in  the  method  of 
erecting  the  cast-iron  lining  rings  by  breaking  joints  with  the 
segments.  In  all  the  iron-lined  tunnels  it  has  been  the  estab- 

O 

lished  custom  to  erect  the  rings  with  continuous  horizontal 
joints.  For  some  reason  it  was  thought  inadvisable  to  attempt 
breaking  joints  with  the  segments.  The  writer's  experience 
in  the  Hudson  tunnel  had  shown  him  the  importance  of  obtain- 
ing, in  soft,  squeezing  ground,  a  perfectly  rigid  tunnel-ring. 
In  a  material  exerting  hydrostatic  pressure  the  tunnel  lining  is 
subjected  to  a  resultant  strain,  tending  to  flatten  the  ring,  or 
decrease  its  vertical  diameter.  Any  yielding  to  this  strain 
results  both  in  increasing  the  deforming  pressure  and  in  de- 
creasing the  power  of  the  ring  to  resist  the  strain.  In  a  lining 
erected  with  continuous  joints  the  rigidity  of  the  ring  is 
dependent  upon  the  bolting  in  the  horizontal  joints.  At  the 
Hudson  River  tunnel  a  ring  of  plates  was  bolted  together 
lying  flat  on  the  ground,  the  plates  all  brought  to  a  true  circle, 
and  the  two  1^-in.  bolts  in  each  joint  well  tightened.  Upon 
raising  this  ring  with  a  derrick,  so  that  it  stood  erect,  the  ring- 


SUBMARINE    TUNNELING  223 

•was  flattened  3  ins.  by  its  own  weight.  At  the  East  River 
tunnel  a  similar  experiment  was  made  ;  two  rings  of  plates 
were  bolted  together,  breaking  joints,  one  ring  being  revolved 
two  holes.  These  two  rings  were  then  raised  upright,  but  no 
flattening  could  be  detected.  By  means  of  a  turnbuckle  a 
measured  strain  was  now  brought  upon  the  rings  along  the 
vertical  diameter.  At  16,000  Ibs.  the  vertical  diameter  was 
shortened  i-in.,  the  flanges  of  the  plates  cracking  where  the 
turnbuckle  was  attached.  In  these  two  instances  there  was,  of 
course,  a  great  difference  in  the  size  of  the  rings,  those  in  the 
Hudson  tunnel  being  18  ft  inside  diameter,  while  those  in  the 
gas  tunnel  were  only  10  ft.  2  ins.  inside  diameter. 

Aside  from  the  rigidity  gained,  breaking  joints  has  proved 
much  the  better  in  other  ways.  With  continuous  joints,  two 
things  are  apt  to  occur:  (1)  The  joint^face  where  two  rings 
meet  may  become  slightly  warped ;  that  is,  all  points  on  this 
face  of  the  ring  will  no  longer  lie  in  the  same  plane.  This 
may  be  caused  by  carelessness  in  allowing  dirt  to  get  into  the 
joints  between  the  rings.  When  this  once  occurs  the  warping 
increases  with  every  additional  ring  till  true  joints  can  no 
longer  be  made.  (2)  The  rings  may  be  erected  so  as  to  depart 
gradually  from  a  true  circular  form.  This  latter  case  is  im- 
possible where  the  joints  are  broken,  and,  in  the  former  in: 
stance,  by  breaking  joints,  the  error  is  divided  and  distributed 
around  the  ring  until  it  disappears. 

On  Jan.  16,  1894,  the  end  of  the  soft  seam  was  reached  with 
the  shield,  and  rock  was  again  entered  after  having  passed 
through  98  ft.  of  soft  ground.  This  rock  resembled  slightly 
the  rock  on  BlackwelTs  Island.  It  was  in  a  much  shattered 
condition,  with  many  loose  heads  and  small,  soft  veins.  As 
this  material  required  support  in  the  heading  and  a  permanent 
lining,  and  as,  in  its  present  condition,  there  was  no  assurance 
that  it  might  not  again  pass  into  soft  material  —  shield  tunnel- 
ing was  still  continued.  Small  machine-drills  were  set  up  in 
the  four  working-chambers  of  the  shield  upon  arms  bolted  to 


224  TUNNELING 

the  vertical  platform,  and  the  rock  was  drilled  and  blasted  just 
ahead  of  the  shield.  The  progress  of  4  ft.  per  day  was  made 
in  this  material,  of  which  there  was  about  65  ft.  The  rock 
then  became  much  more  solid,  with  a  roof  that  was  self-sustain- 
ing, and  arrangements  were  made  for  removing  the  shield. 
On  Feb.  18  the  work  of  removing  the  shield  was  begun,  and 
two  days  later  everything  was  ready  for  the  regular  rock-tunnel 
work  in  the  heading,  the  shield  having  been  taken  apart  and 
removed  in  that  time. 

At  about  the  time  that  shield  tunneling  was  being  discon- 
tinued at  New  York,  it  was  being  installed  at  Long  Island. 
An  entire  duplicate  plant  had  been  ordered  for  this  side ;  for, 
although  it  had  been  originally  intended  to  use  one  shield  for 
both  headings,  it  was  later  deemed  advisable  to  provide  a  shield 
for  each  heading,  so  that  there  might  be  no  delay,  should  soft 
ground  be  met  in  both  headings  at  the  same  time.  In  passing 
through  the  soft  seam  at  Ravenswood  with  the  shield,  no 
especial  difficulties  were  met.  -  The  material  proved  to  be  a 
mass  of  soft-rock  fragments,  boulders  and  cinder-like  stones  im- 
bedded in  soft  green  chlorite.  About  a  month  was  consumed 
in  passing  through  this  seam,  removing  the  shield,  and  prolong- 
ing the  cast-iron  lining  well  into  the  rock  on  both  sides  of 
the  vein.  With  both  tunnel  headings  now  in  rock,  remarkably 
rapid  progress  was  made ;  and  as  progress  now  had  become  of 
great  importance  to  the  company,  a  liberal  bonus,  arranged  on 
a  sliding  scale,  was  given  the  foremen  for  work  done  over  stated 
amounts.  Up  to  the  time  of  the  headings  meeting,  an  average 
progress  of  69  ft.  per  week  was  made,  while  in  rock,  on  both 
the  New  York  and  Long  Island  sides.  The  record  week  of 
the  work  was  the  one  ending  June  27,  when  at  Ravenswood 
95  ft.  was  driven,  while  on  the  New  York  side,  the  heading 
was  advanced  101  ft.,  making  a  total  for  the  week  of  196  ft.  of 
tunnel  driven.  Soon  after  the  rock  tunneling  had  been  re- 
sumed on  the  New  York  side,  this  heading  reached  Blackwell's 
Island,  and  the  troubles  on  this  side  were  over.  But  at  Ravens- 


SUBMARINE   TUNNELING  225 

wood,  with  the  heading  in  white  limestone,  there  was  every 
reason  to  expect  further  soft  seams  where  the  rock  should 
change  to  the  granite  gneiss  of  Blackwell's  Island.  These 
expectations  were  not  disappointed ;  for  after  passing  through 
350  ft.  of  the  limestone,  and  when  within  200  ft  of  Blackwell's 
Island,  a  soft  seam  was  met,  and  air  pressure  had  to  be  once 
more  used  in  the  heading.  As  this  seam  was  but  14  ft.  in 
width,  and  presented  no  especial  difficulties,  the  tunnel  was 
carried  through  it  without  using  the  shield,  the  cast-iron  seg- 
ments being  erected  under  a  timber  roof.  Gneiss  was  encoun- 
tered on  the  other  side  of  this  soft  vein,  which  brought  with  it 
the  assurance  that  the  last  of  the  soft  ground  had  been  passed. 
On  May  16  serious  loss  and  delay  were  caused  by  a  fire  which 
destroyed  the  New  York  works.  The  fire  started  in  an  adjoin- 
ing picnic  ground,  containing  many  light  frame  structures, 
which  caused  so  fierce  a  conflagration  that  it  was  impossible  to 
save  our  works.  This  caused  a  delay  of  three  weeks  in  the 
time  of  the  tunnel's  completion.  On  July  11,  1894,  the  re- 
maining 15  ft.  of  rock  between  the  headings  was  blasted  away, 
thus  opening  the  pioneer  tunnel  under  the  East  River,  two 
years  from  the  time  when  ground  was  first  broken.  Some 
weeks  were  spent  in  clearing  up  and  shutting  out  the  water  in 
the  wet  places.  A  3-ft.  gas  main  was  now  laid  through  to 
New  York,  and  on  Oct.  15  gas  was  delivered  into  the  city, 
accomplishing  the  purpose  of  the  tunnel. 

VAN  BUREN  STREET  TUNNEL,   CHICAGO. 

The  Van  Buren  Street  tunnel  in  Chicago  belongs  to  that 
class  of  submarine  tunnels  which  has  been  designated  as 
tunnels  on  the  river  bed,  by  which  it  is  meant  that  the  top  of 
the  tunnel  is  flush  with,  or  extends  slightly  above,  the  bed  of 
the  stream.  Two  methods  are  available  for  constructing  these 
tunnels ;  viz.,  the  cofferdam  method  and  the  caisson  method. 
The  cofferdam  method  has  been  actually  employed  in  several 


226  TUNNELING 

instances ;  but  the  caisson  method,  although  proposed  for  sev- 
eral projected  works,  has  never  actually  been  employed.  The 
Van  Buren  Street  tunnel,  built  to  carry  a  double-track  street 
railway  under  the  Chicago  River,  was  completed  in  1894  by 
the  cofferdam  method.  The  special  features  of  the  tunnel  * 
are :  (1)  the  unusually  large  dimensions  of  the  cross-section  of 
30ft.  X  15ft.  9  ins.;  (2)  its  construction  inside  of  coffer- 
dams of  great  length  and  wdith;  (3)  the  construction  under 
some  very  high  buildings  calling  for  great  care  and  very  strong 
temporary  and  permanent  supports. 

The  special  feature  of  the  work  for  our  present  purpose 
was  the  construction  of  the  tunnel  across  the  river.  To  accom- 
plish this  a  cofferdam  was  built  out  from  the  west  shore  of  the 
river  to  its  middle,  and  the  tunnel  constructed  within  it  like 
the  building  of  any  other  structure  within  a  cofferdam.  Trans- 
verse and  longitudinal  sections  of  this  cofferdam  are  shown  by 
Fig.  119.  As  will  be  seen,  it  was  a  simple  double-wall  coffer- 
dam, with  a  clear  width  between  the  walls  of  58  ft.,  and  braced 
transversely  as  shown.  Inside  of  this  a  single-wall  cofferdam 
of  piles  was  constructed,  with  a  clear  width  just  sufficient  to 
allow  the  construction  of  the  masonry  within  it.  When  the 
tunnel  end  reached  the  channel  end  of  the  cofferdam,  a  crib-wall 
was  built  over  the  end  of  the  completed  tunnel,  as  shown  by 
the  drawings.  This  crib  wall  was  intended  to  form  the  end 
wall  of  another  cofferdam,  which  was  built  out  from  the  east 
shore,  and  within  which  the  remaining  half  of  the  tunnel  was 
built  as  the  first  half  had  been.  The  drawings  show  the  char- 
acter of  the  tunnel  masonry  and  of  the  centering  upon  which 
it  was  built. 

In  this  connection  it  will  be  interesting  to  mention  briefly 
the  most  pretentious  proposition  for  tunnel  construction  by 
means  of  caissons.  Some  years  ago,  Prof.  Winkler  proposed 
to  construct  a  tunnel  under  the  River  Danube  to  connect  the 
various  portions  of  the  Vienna,  Austria,  underground  railway, 

*  "  Eng.  News,"  April  12, 1892. 


SUBMARINE   TUNNELING 


227 


228  TUNNELING 

and  to  use  caissons  in  the  construction.  Prof.  Winkler  pro- 
posed to  build  caissons  from  30  ft.  to  45  ft.  long,  with  a  width 
depending  upon  the  lateral  dimensions  adopted  for  the  tunnel 
masonry.  The  caisson  was  to  be  made  of  metal  plates  and 
angle  iron  with  riveted  connections  on  all  sides  except  those 
running  vertically  transverse  to  the  tunnel  axis,  whose  connec- 
tions were  to  be  bolted.  The  roof  of  the  caisson  was  to  be 
made  of  T-irons  resting  upon  templates  placed  on  the  edge 
of  the  longitudinal  sides  of  the  caisson,  and  strutted  in  the 
middle  by  the  crown  of  an  iron  arch  having  its  springers  upon 
brackets  inserted  on  the  vertical  angle  irons  forming  the  frame 
of  the  caisson.  Between  the  T-irons  of  the  roof  small  brick 
vaults  were  to  be  built,  and  a  very  thick  stratum  of  concrete 
laid  on  their  extrados  so  as  to  obtain  a  level  surface.  In  the 
middle  of  the  roof  an  opening  was  to  be  left ;  this  was  for  the 
shaft  having  the  air-locks  to  allow  the  passage  of  men,  mate- 
rials, and  compressed  air. 

Across  the  river  two  parallel  rows  of  piles  were  to  be 
driven  into  the  river  bed,  to  fix  the  place  where  the  caisson  was 
to  be  sunk.  Then  the  first  caisson  near  the  shore  was  to  be 
lowered  in  the  ordinary  way,  and  a  second  caisson  was  to  be 
immediately  sunk  very  close  to  the  first  one.  When  both  cais- 
sons had  reached  the  plane  of  the  tunnel  floor,  the  sides  which 
were  in  contact  were  to  be  unbolted  and  removed,  and  the 
small  space  between  made  water-tight  by  filling  them  with  yarn 
and  tar.  The  chambers  of  the  two  caissons  were  to  be  opened 
into  a  single  large  one  communicating  above  by  means  of  two 
shafts.  At  the  same  time  that  the  masonry  was  being  built  in 
the  two  first  caissons,  from  the  inverted  arch  up,  a  third  cais- 
son was  to  be  sunk ;  and  when  by  excavation  it  had  reached 
the  plane  of  the  projected  tunnel  floor,  the  partitions  were 
to  be  removed  so  that  the  three  caissons  were  in  communica- 
tion, forming  a  large  single  caisson.  To  limit  the  compressed 
air  to  the  working-place,  walls  were  to  be  built  across  the  tun- 
nel near  the  advanced  part  completely  lined.  The  first  wall 


SUBMARINE   TUNNELING  229 

was  to  be  built  after  four  caissons  were  sunk.  Then  the  outer 
partition  of  the  first  caisson  was  to  be  removed,  and  the  ma- 
sonry of  the  submarine  tunnel  connected  with  the  portion  of 
the  tunnel  built  on  land.  In  a  similar  manner  all  the  caissons 
were  to  be  sunk ;  and  when  the  last  one  was  placed,  and  the  ma- 
sonry lining  constructed,  and  connected  with  the  portion  of 
the  tunnel  built  on  the  other  shore  of  the  river,  the  partition 
walls  were  to  be  battered  down,  and  the  submarine  tunnel  com- 
pletely constructed  and  open  to  traffic. 


230  TUNNELING 


CHAPTER  XX. 

SUBMARINE    TUNNELING    (Continued). —THE 
MILWAUKEE  WATER-WORKS  TUNNEL. 


THE  new  water  supply  intake  tunnel  for  the  city  of  Mil- 
waukee, Wis.,  is  one  of  the  most  difficult  examples  of  tunnel 
construction  which  American  engineering  practice  has  afforded. 
The  difficulties  were  in  a  large  measure  unexpected  when  the 
work  was  decided  upon  and  put  under  way.  The  tunnel  began 
and  ended  in  a  hard,  impervious  clay,  practically  a  rock,  and 
all  the  preliminary  investigations  led  to  the  conclusion  that 
the  same  favorable  material  would  be  encountered  for  its 
entire  length.  With  such  material  a  brick-lined  tunnel  7^  ft. 
in  diameter  presented  no  unusual  problems ;  but  after  about 
1,640  ft.  had  been  excavated  from  the  shore  end  the  tunnel 
ran  out  of  the  hard  clay,  and  for  the  next  600  ft.  or  more 
a  variety  of  water-bearing  material  was  encountered,  which 
tried  the  skill  and  patience  of  the  engineer  to  their  utmost. 
Other  difficulties  were  indeed  met  with,  but  these  were  of  minor 
importance  in  comparison  with  that  of  safely  and  successfully 
penetrating  the  water-bearing  drift. 

The  work  of  sinking  the  shore  shafts  and  excavating  the 
first  1,600  ft.  of  tunnel  did  not  prove  especially  difficult.  A 
hard,  compact,  and  rock-like  clay,  bearing  very  little  moisture, 
was  encountered  all  along,  and  was  blasted  and  removed  in  the 
ordinary  manner.  The  only  mishap  which  occurred  with  this 
portion  of  the  work  was  the  destruction  of  the  contractor's 
boiler  plant  by  fire  on  Jan.  12,  1891,  which  allowed  the  tunnel 
to  fill  with  water,  and  delayed  work  about  a  month.  By 
Oct.  21,  1891,  1,640  ft.  had  been  driven,  averaging  about  6§  ft. 


SUBMARINE    TUNNELING  231 

per  day,  all  in  the  hard  clay.  No  timbering  had  been  necessary, 
and  except  for  the  first  100  ft.  of  the  tunnel  there  was  very 
little  seepage.  On  the  afternoon  of  Get  21  water  was  observed 
coming  out  from  one  of  the  diill  holes  in  the  heading, 'but  no 
attention  was  paid  to  it.  Shortly  after  a  blast  was  fired,  and 
was  immediately  followed  by  a  rush  of  water  from  the  heading. 
An  unsuccessful  attempt  was  made  to  check  the  flow,  and  the 
pumps  were  started ;  but  they  were  unable  to  keep  the  water 
down,  and  after  seven  hours'  hard  work  the  tunnel  was  aban- 
doned. By  the  next  morning  the  tunnel  and  shaft  were  full  of 
water. 

Several  attempts  were  made  to  empty  the  tunnel ;  but  the 
limited  pumping  capacity  was  not  equal  to  the  task,  and  it  was 
finally  decided  to  install  larger  pumps.  The  pumping  had,  how- 
ever, shown  that  about  1,000  gallons  of  water  a  minute  was 
coming  through  the  leak.  With  the  increased  pumping  plant 
the  tunnel  was  finally  laid  dry  Feb.  13,  1892.  Upon  examina- 
tion the  head  of  the  drift  was  found  to  be  in  the  same  undis- 
turbed condition  in  which  it  was  left  when  the  water  broke  in 
three  months  before. 

A  brick  bulkhead  was  built  into  the  end  of  the  brickwork 
of  the  tunnel,  and  provided  with  a  timber  door  for  passage,  and 
two  10-in.  pipes  for  the  outlet  of  the  water.  With  these  open- 
ings closed,  the  flow  was  checked  sufficiently  to  allow  the  pla- 
cing of  pumps  at  the  bottom  of  the  shore  shaft.  Meanwhile  the 
pressure  of  the  water  against  the  bulkhead  caused  dangerous 
leakage,  and  so  after  the  pumps  were  in  position  the  10-in.  pipes 
were  opened,  relieving  the  pressure  and  allowing  the  water  its 
normal  rate  of  flow.  Trouble  with  the  pumps  now  arose,  and 
after  various  stoppages  and  breaks  the  discharge  pipe  finally 
fell,  disabling  the  whole  plant.  It  became  necessary  to  close 
the  10-in.  pipes  in  the  bulkhead  and  draw  up  the  pumps.  This 
allowed  the  tunnel  to  again  fill  with  water. 

After  thoroughly  overhauling  the  pumping  machinery,  the 
contractor  again  laid  the  tunnel  dry  on  March  19;  and  after 


232 


TUNNELING 


the  pumps  had  been  permanently  placed  so  as  to  take  care  of 
the  water,  an  examination  of  the  work  was  made.  It  was  found 
that  the  water  was  coming  from  the  north,  and  with  the  hope 
of  avoiding  the  difficulties  of  the  old  heading,  it  was  decided  to 
make  a  detour  of  the  south.  On  April  16  work  was  begun  at 
a  point  about  90  ft.  back  from  the  face,  and  deflecting  the  line 
about  38°  toward  the  south.  About  38  ft.  from  the  angle  of 
junction  a  brick  bulkhead  with  two  8-in.  openings  was  built 


;jr  ~v  f VV%:  , 

FlG.  120.  —  Sketch  showing  underground  stream.  Milwaukee  Water- Works  Tunnel. 

into  the  new  bore.  The  work  progressed  successfully  for  about 
75  ft.,  when  water  was  again  encountered ;  and  upon  pushing 
forward  the  heading,  gravel  and  sand  came  in  such  quantities 
that  it  was  found  impracticable  to  continue  the  work  further. 
On  June  1  the  bulkhead  was  permanently  closed,  and  the  work 
in  this  direction  was  abandoned. 

A  further  and  closer  examination  was  now  made  of  the 
heading  first  abandoned.  Upon  breaking  through  the  rock-like 
clay  it  was  found  that  the  water  came  from  an  underground 


SUBMARINE   TUNNELING  233 

stream  flowing  from  the  north  through  a  well  defined  channel 
in  red  clay.  This  channel  was  about  13  ft.  above  the  grade  of 
the  tunnel ;  and  above  it  in  every  direction  visible  was  a  bed  of 
hard,  dry,  red  clay,  while  immediately  in  front  of  the  face  of  the 
work  was  a  bank  of  coarse  gravel.  Fig.  120  is  a  sketch  of  the 
channel  and  stream  where  they  entered  the  work.  In  this  last 
drawing  the  photograph  has  been  followed  exactly,  no  particu- 
lar being  exaggerated  in  the  slightest.  The  water  from  this 
stream  was  clear  and  pure;  and  a  chemical  analysis  showed 
that  it  was  not  lake  water,  but  must  come  from  some  separate 
source. 

While  the  engineer  did  not  consider  the  difficulty  of  pro- 
ceding  along  the  old  Line  insurmountable,  it  was  decided  to  be 
less  difficult  on  the  whole  to  go  back  from  150  ft.  to  175  ft  and 
deflect  the  line  to  the  north  and  upward,  so  as  to  pass  over  the 
underground  entrance.  Instead  of  allowing  the  water  to  flow 
at  its  normal  rate  and  take  care  of  it  by  pumping,  the  contrac- 
tors desired  to  reduce  the  pumping,  and  to  this  end  they  con- 
structed a  bulkhead  just  west  of  the  deflection  toward  the 
south  with  a  view  of  shutting  off  the  water.  The  water,  how- 
ever, accumulated  with  a  pressure  of  some  50  Ibs.  per  sq.  in., 
and  penetrated  the  filling  around  the  brick  lining  of  the  tunnel, 
preventing  the  cutting  through  of  the  lining  for  the  new  line. 
A  second  bulkhead  was  then  built  about  20  ft.  west  of  the 
first,  but  with  not  much  better  results,  for  upon  closing  it  the 
water  was  found  to  leak  through  the  brickwork  for  a  long 
distance  west.  Finally  on  Aug.  2,  1892,  the  contractors 
lifted  their  pumps  and  allowed  the  tunnel  to  fill  again  with 
water. 

No  further  work  was  done  on  the  tunnel  by  the  contractors, 
although  they  continued  work  on  the  lake  shaft  for  some 
months.  Difficulties  had,  however,  arisen  here,  which  will  be 
described  further  on ;  and  finally  a  disagreement  arose  between 
the  contractors  and  the  city  over  the  delay  in  prosecuting 
the  tunnel  work  and  over  one  or  two  other  questions,  which 


234  TUNNELING 

resulted  in  the  City  Council  suspending  their  contract  and 
ordering  the  Board  of  Public  Works  to  go  ahead  with  the 
work. 

The  first  step  to  be  taken  by  the  engineer  was  to  purchase 
adequate  pumping  machinery  and  empty  the  tunnel.  This  was 
effected  Jan.  17, 1894  ;  and  as  soon  as  practicable  thereafter  the 
two  bulkheads  were  removed  and  the  tunnel  cleaned,  tram-car 
tracks  laid,  and  everything  prepared  for  work.  It  was  now 
determined  to  go  ahead  on  the  original  line  of  the  tunnel  if 
possible,  and  the  bulkhead  here  was  removed  and  work  begun. 
Meanwhile,  a  safety  bulkhead  had  been  built  to  replace  the  first 
one  torn  away.  This  was  provided  with  a  door  and  drain- 
age pipes.  Work  was  begun  on  the  original  heading,  but  had 
proceeded  only  a  little  way  when  the  water  broke  in,  driving 
out  the  workmen.  This  was  removed  three  or  four  times,  when 
the  flow  suddenly  increased  to  3,000  gallons  per  minute.  An 
examination  of  the  lake  bottom  above  the  break  showed  that  it 
had  settled  down,  indicating  that  the  new  break  connected  with 
the  lake  bottom,  and  making  further  work  along  the  original 
line  out  of  the  question. 

The  question  now  arose  what  it  was  best  to  do.  It  was 
impracticable  to  use  a  shield,  as  the  material  ahead  of  the  break 
required  blasting,  and  the  pressure  from  above  was  enormous. 
On  account  of  its  expense  and  difficulty  of  application  the 
freezing  process  did  not  seem  advisable,  and  the  plenum  process 
was  likewise  out  of  the  question  on  account  of  the  great 
pressure  which  would  be  required  at  this  depth.  The  detour 
to  the  south  which  had  been  made  by  the  contractor  had  been 
unsuccessful,  and  had  left  the  ground  in  a  treacherous  condi- 
tion. To  depress  the  tunnel  was  not  advisable,  for  it  was  not 
by  any  means  certain  that  the  bed  of  gravel  could  be  avoided 
in  that  way ;  and,  moreover,  it  would  be  necessary  to  ascend 
again  further  on,  and  thus  leave  a  trap  which  would  effectually 
cut  off  escape  to  those  at  work  on  the  face  if  water  again  broke 
into  the  tunnel. 


SUBMARINE    TUNNELING  235 

It  was  finally  decided  that  the  old  plan  of  deflecting  the 
line  toward  the  north  and  upward  so  as  to  pass  over  the  under- 
ground stream  should  be  tried.  A  hole  was  therefore  cut 
through  the  tunnel  lining  1,433  ft.  from  the  shore,  and  work 
was  begun  on  a  detour  of  20°  toward  the  north  and  an  upward 
grade  of  10  %.  Fair  progress  was  made  on  this  new  line, 
gradually  ascending  into  solid  rock,  until  May  10,  when  the 
test  borings,  which  were  constantly  made  in  every  direction 
from  the  face,  showed  that  sand  was  being  approached.  A 
brick  bulkhead  was  therefore  built  into  the  masonry  as  a  safe- 
guard, should  it  happen  that  water  was  encountered  in  large 
quantities.  As  the  borings  seemed  to  indicate  that  the  top 
surface  of  the  rock  underlying  the  sand  was  nearly  level,  the 
lower  half  of  the  tunnel  was  first  excavated,  leaving  about  18 
ins.  of  the  rock  to  serve  as  a  roof  (Sketch  a,  Fig.  121),  and  the 
brick  invert  was  built  for  a  distance  of  52  ft.  The  rock  roof 
was  then  carefully  broken  through  for  short  distances  at  a  time, 
and  short  sheeting  driven  ahead  into  the  sand,  which  proved  to 
be  a  very  fine  q/iicksand  flowing  through  the  smallest  openings. 
Extreme  care  had  to  be  taken  in  this  work,  but  little  by  little 
the  brickwork  was  pushed  ahead  until  at  a  distance  of  90  ft. 
from  the  point  where  the  sand  was  first  net,  and  208  ft.  from 
the  old  tunnel,  the  sand  stopped  and  the  heading  entered  a 
hard  clay. 

All  this  work  had  been  done  on  an  ascending  grade,  and  the 
ascent  was  continued  about  40  ft.  farther  in  the  clay.  By  this 
time  a  sufficient  elevation  was  gained  to  pass  over  the  under- 
ground stream,  and  the  tunnel  line  was  changed  to  head  toward 
the  lake  shaft,  and  the  grade  reduced  to  a  level.  The  under- 
ground stream  was  passed  without  trouble  and  the  tunnel 
continued  for  a  distance  of  54  ft.  without  difficulty.  On  July 
10  the  clay  in  the  heading  suddenly  softened,  and  before  the 
miners  could  secure  it  by  bracing,  the  water  rushed  in,  followed 
by  gravel,  filling  up  solidly  some  34  ft.  of  the  tunnel  before  it 
was  stopped  by  a  timber  bulkhead  hastily  built. 


236 


TUNNELING 


Upon  examining  the  lake  bottom  a  cavity  over  60  ft.  deep  and 
10  ft.  in  diameter  was  found  directly  over  the  end  of  the  tunnel, 
which  had  been  caused  by  the  gravel  breaking  into  the  tunnel. 
Having  now  reached  an  elevation  where  it  was  possible  to  use 
compressed  air,  it  was  determined  to  put  in  double  air-locks 
and  use  the  plenum  process.  The  locks  were  built,  and  some 


r.  n-  / * 

Bench  Face,     '  •$ 
Packed  with  Clay.  ,* 
I  «? 


Longitudinal  Section  Showing  Method  of    \'H\  /- 
Construction  in  Rock  Covered  with  Quicksand.        »      ^^ 

Sketch  "a". 


Section  A-B-C-D. 
Sketch  "c". 


Cross  Section  Showing  Manner  of 
Longitudinal  Section  at  Tunnel .  Constructing  Lining  around  Boulder. 

Sketch  ttb".  Sketch  "d." 

FlG.  121.—  Sketch  Showing  Methods  of  Lining,  Milwaukee  Water- Works  Tunnel. 

670  cu.  yds.  of  clay  were  dumped  into  the  hole  in  the  lake 
bottom.  On  Aug.  4  the  air-locks  were  tried  with  26  Ibs.  air 
pressure ;  but,  upon  a  temporary  release  of  the  pressure,  the 
water  passed  around  the  locks  and  back  of  the  tunnel  lining 
for  some  distance,  and  even  forced  through  the  lining,  carrying 
considerable  clay  and  fine  sand  with  it.  Upon  sounding  the 


SUBMARINE   TUNNELING.  237 

lake  bottom  it  was  found  that  the  cavity  had  again  increased 
to  a  depth  of  65  ft,  whereupon  an  additional  600  cu.  yds.  of 
clay  were  dumped  into  it. 

On  account  of  the  water  leaking  through  the  brickwork,  the 
only  dry  place  to  cut  through  the  brickwork  and  build  in  an 
air-lock  was  just  ahead  of  the  brick  bulkhead.  This  lock  wa& 
completed  Aug.  27,  and  to  avoid  encountering  the  danger  of 
the  direct  connection  with  the  lake  at  the  end  of  the  drift,  it 
was  decided  to  make  another  detour  to  the  north.  On  Aug.  28r 
therefore,  the  brick  on  the  north  side  of  the  tunnel  12  ft.  back 
from  the  end  of  the  brickwork  was  cut  through  under  25  Ibs. 
air  pressure,  and  work  proceeded  in  good,  hard  clay.  The 
original  air-lock  was  cut  out  and  a  new  lock  built  into  this 
clay  about  34  ft.  from  the  last  detour,  to  be  used  in  case  of 
further  difficulties.  After  building  the  tunnel  for  about  80  ft. 
from  the  detour,  the  soundings  again  indicated  the  approach  to 
gravel  and  water,  and  on  Oct.  14  the  water  broke  through  from 
the  bottom  in  such  volume  and  with  such  force  that  the  men 
ran  out,  closing  every  air-lock  and  the  valves  of  every  drain  in 
their  haste  to  escape,  until  the  brick  bulkhead  was  reached. 
It  was  with  great  difficulty  that  the  portion  of  the  tunnel  up  to 
the  last  air-lock  was  recovered  and  cleaned  out. 

It  was  now  recognized  that  a  pressure  of  from  38  to  40  Ibs. 
of  air  would  be  needed  to  hold  this  water,  and  accordingly  an- 
other compressor  was  added  to  the  plant.  With  a  pressure  of 
36  Ibs.  the  water  was  driven  out  and  the  work  again  started. 
At  this  time  also  an  additional  350  cu.  yds.  of  clay  were  dumped 
into  the  hole  in  the  lake  bottom.  Altogether,  1,620  cu.  yds. 
of  clay  had  been  put  into  this  hole. 

Loose  gravel  and  boulders,  some  of  immense  size,  were  now 
encountered,  and  the  work  became  exceedingly  difficult  on 
account  of  the  great  escape  of  air.  The  interstices  between  the 
gravel  and  boulders  were  not  filled  with  silt  or  sand,  but  con- 
tained water.  Moreover,  this  material  extended  upward  to  the 
lake  bottom,  as  was  shown  by  the  escape  of  air  at  the  surface  of 


288  TUNNELING 

the  lake.  For  an  area  of  several  hundred  square  feet  the  surface 
of  the  water  resembled  a  pot  of  boiling  water.  At  times  the 
air  would  escape  very  rapidly,  and  again  only  a  few  bubbles 
would  show. 

It  need  hardly  be  said  that  the  work  in  this  gravel  was  very 
slow.  It  was  impossible  to  blast  or  to  tear  out  the  large  boulders 
whole,  as  so  much  surface  would  be  exposed  that  an  inrush  of 
water  would  take  place  despite  the  air  pressure.  The  method 
of  procedure  was  to  excavate  a  heading  and  build  the  brick  roof 
arch  first,  and  then  to  take  out  the  bench  and  build  the  in- 
vert. Fig.  121  gives  a  number  of  sketches  showing  how  the 
work  was  done.  A  short  piece  of  heading  was  taken  out,  the 
top  and  face  of  the  bench  being  meanwhile  plastered  with  clay 
(Sketches  b  and  c,  Fig.  121)  to  reduce  the  escape  of  air,  and 
then  the  roof  arch  was  built  and  supported  on  side  sills  resting 
on  the  bench.  Bit  by  bit  the  roof  arch  was  pushed  forward 
until  some  little  distance  had  been  completed,  then  the  heading 
was  plastered  with  clay  and  the  bench  taken  out  little  by  little 
and  the  invert  built.  All  the  gravel  except  the  small  area 
upon  which  work  was  actually  in  progress  was  kept  thoroughly 
plastered  with  clay ;  and  as  the  air  escaped  through  the  com- 
pleted brick  work  very  rapidly,  water  was  allowed  to  cover  a 
portion  of  the  invert  (see  Sketch  c,  Fig.  121),  so  as  to  reduce 
the  area  of  escape. 

When  a  large  boulder  was  reached,  which  lay  partly  within 
and  partly  without  the  tunnel  section,  the  lining  was  built  out 
and  around  it,  as  shown  in  Sketch  d,  Fig.  121.  The  boulder 
was  then  broken  and  taken  out.  All  through  this  gravel  bed 
the  cross-section  of  the  lining  is  made  irregular  by  the  con- 
struction of  these  pockets  in  the  lining  to  get  around  boulders. 
Sometimes  they  were  on  one  side  and  sometimes  on  the  other, 
or  on  both,  or  at  the  top  or  bottom.  In  fact,  there  was  no 
regularity.  Despite  the  hazard  and  danger  of  this  work,  con- 
tinual progress  was  made,  though  sometimes  it  was  only  4  ft. 
of  completed  tunnel  per  week,  working  night  and  day ;  and,  if 


SUBMARINE    TUKXEL1XG  239 

some  cases  of  caisson  disease  be  excepted,  the  only  mishap  oc- 
curring was  a  fire  which  got  into  the  timber  packing  behind 
the  lining  and  caused  some  trouble.  From  the  gravel  the  tunnel 
ran  into  clay  and  quicksand,  and  then  into  hard,  dry  clay 
similar  to  that  encountered  near  the  shore.  Some  difficulty 
was  had  with  the  quicksand,  but  it  was  successfully  overcome  : 
and  when  the  hard  clay  was  struck,  the  trouble,  as  far  as  the 
work  from  the  shore  shaft  was  concerned,  was  virtually  over. 

Meanwhile,  a  different  set  of  afflictions  had  come  upon  the 
engineer  and  contractors  in  sinking  the  lake  shaft  and  driving 
the  heading  toward  shore.  This  shaft  was  intended  to  be 
built  by  sinking  a  cast-iron  cylinder  10  ft.  in  diameter,  made 
up  of  sections  bolted  together.  Work  was  begun  July  5,  1892, 
and  the  sinking  was  accomplished  first  by  weighting  the  cylinder, 
and  afterwards  by  pumping  out  the  sand  and  water  within  it 
until  the  pressure  from  the  outside  broke  through  under  the 
cutting  edge  and  forced  the  sand  into  the  cylinder,  allowing  it 
to  sink  a  little.  From  10  to  30  cu.  yds.  of  sand  were  carried 
into  the  cylinder  each  time,  and  finally  it  was  feared  that  if 
the  process  continued,  the  crib,  which  had  been  previously 
erected,  would  be  undermined.  On  Sept.  6,  therefore,  the 
contractors  were  ordered  to  discontinue  this  method  of  work. 
No  change  was  made,  however,  until  Oct.  1,  when  the  cylinder 
had  reached  a  depth  of  68  ft.,  and  by  this  time  there  was  quite 
a  large  cavity  underneath  the  crib.  This  was  refilled,  and  the 
cylinder  pumped  out,  and  excavation  begun  inside  of  it.  On 
Oct.  11  a  2^-ft.  deep  ring  of  brick  work  was  laid  underneath 
the  cutting  edge  ;  but  in  trying  to  put  in  another  ring  beneath 
the  first,  two  days  later,  the  sand  and  water  broke  through  the 
bottom,  driving  the  men  out,  and  filling  the  cylinder  to  a  depth 
of  16  ft.  with  sand.  The  pumps  w^ere  started,  but  the  water 
could  not  be  lowered  to  a  greater  depth  than  60  ft. 

At  the  request  of  the  contractors,  the  city  engineer  had  a 
boring  made  at  the  center  of  the  shaft  to.  determine  the 
character  of  the  material  to  be  further  penetrated.  This 


240  TUNNELING 

boring  showed  that  sand  mixed  with  loam  and  gravel  would  be 
found  for  a  depth  of  26  ft.,  then  would  come  15  ft.  of  red  clay, 
and  finally  a  layer  of  hard  clay  like  that  penetrated  by  th& 
shore  end  of  the  tunnel.  About  the  middle  of  December  the 
contractors  made  another  attempt  to  pump  the  shaft,  but  find- 
ing  that  the  water  came  in  at  the  rate  of  25  gallons  a  minute,, 
abandoned  the  attempt.  In  the  latter  part  of  February  prepa- 
rations were  made  to  put  an  air-lock  in  the  shaft  and  use 
compressed  air.  Hardly  had  the  work  been  begun  by  this 
system,  when,  on  April  20, 1893,  a  terrific  easterly  storm  swept 
the  top  of  the  crib  bare  of  the  buildings  and  machinery,  and 
drowned  all  but  one  of  the  15  men  at  work  there. 

This  disaster  delayed  the  work  for  some  time,  but  in  June 
the  contractors  erected  a  new  building  and  new  machinery,  and 
resumed  work.  Very  little  progress  was  made ;  and  the  air  es- 
caped so  rapidly  that  it  loosened  the  sand  surrounding  the 
shaft  and  reduced  the  friction  to  such  an  extent  that  on  July 
28  the  entire  cylinder  lifted  bodily  about  6  ft.,  and  sand  rushed 
in,  filling  the  lower  part  of  the  cylinder  to  within  45  ft.  of  the 
lake  surface.  No  further  work  was  done  by  the  contractors, 
although  they  submitted  a  proposition  to  sink  a  steel  cylinder 
inside  the  cast-iron  cylinder  and  extending  from  5  ft.  above 
datum  to  100  ft.  below  datum  for  $300  per  ft.  This  proposi- 
tion was  refused  by  the  city;  and  since  work  on  the  tunnel 
proper  has  been  abandoned  by  the  contractors  some  time  before, 
as  had  already  been  described,  the  city  suspended  their  contract 
on  Oct.  19. 

On  Oct.  30  a  contract  was  made  with  Mr.  Thos.  Murphy, 
of  Milwaukee,  Wis.,  to  sink  a  steel  cylinder  inside  the  old  iron 
cylinder.  The  water  was  first  pumped  out  of  the  old  cylinder, 
and  a  timber  bulkhead  built  at  the  bottom.  On  this  the  steel 
cylinder  was  built,  and  then  the  bulkhead  was  removed.  Air 
pressure  was  put  on,  and  the  excavation  proceeded  successfully 
until  the  bottom  layer  of  clay  was  met  with,  when  all  chances 
for  trouble  ceased. 


SUBMARINE   TUNNELING  241 

The  cylinder,  as  it  was  completed,  penetrated  9  ft.  into  the 
hard  clay,  and  was  underpinned  with  brickwork  for  a  depth  of 
29  ft.  or  more,  to  a  point  4  ft.  below  the  grade  line  of  the 
tunnel.  At  the  lower  end,  the  section  of  the  shaft  was  changed 
from  a  circle  to  a  square.  Later  the  steel  cylinder  was  lined 
with  brick. 

On  March  28,  1894,  an  agreement  was  made  with  Mr. 
Thos.  Murphy  to  construct  the  tunnel  from  the  lake  shaft 
toward  the  shore.  Except  that  considerable  water  was  en- 
countered, which,  owing  to  inadequate  pumping  machinery, 
filled  the  tunnel  and  shaft  at  two  different  times,  and  had  to 
be  removed,  no  very  great  difficulty  was  had  with  this  part  of 
the  work. 

On  July  28,  1895.  the  headings  from  the  lake  and  shore 
shafts  met.  Meanwhile  the  cast-iron  pipe  intake,  the  intake 
crib,  etc.,  had  been  completed,  and  practically  all  that  remained 
to  be  done  was  to  clean  the  tunnel  and  lift  the  pumping 
machinery  at  the  shore  shaft.  During  the  cleaning,  the  air 
pressure  had  been  kept  up  on  account  of  the  leakage  through 
the  brick  lining,  and,  indeed,  the  pressure  was  kept  up  until 
the  last  possible  moment,  and  everything  made  ready  for 
removing  the  air  locks,  bulkheads,  pumps,  etc.,  in  the  least 
possible  time.  The  pumps  were  the  last  to  come  out. 


242  TUNNELING 


CHAPTER   XXL 

SUBMARINE    TUNNELING     (Continued).  —  THE 
SHIELD  SYSTEM. 


Historical  Introduction.  —  The  invention  of  the  shield  system 
of  tunneling  through  soft  ground  is  generally  accredited  to  Sir 
Isambard  Brunei,  a  Frenchman  born  in  1769,  who  emigrated  to 
the  United  States  in  1793,  where  he  remained  six  years,  and 
then  went  to  England,  in  which  country  his  epoch-making  in- 
vention in  tunneling  was  developed  and  successfully  employed  in 
building  the  first  Thames  tunnel,  and  where  he  died  in  1849,  a 
few  years  after  the  completion  of  this  great  work.  Sir  Isambard 
is  said  to  have  obtained  the  idea  of  employing  a  shield  to  tunnel 
soft  ground  from  observing  the  work  of  ship-worms.  He  no- 
ticed that  this  little  animal  had  a  head  provided  with  a  boring 
apparatus  with  which  it  dug  its  way  into  the  wood,  and  that  its 
body  threw  off  a  secretion  which  lined  the  hole  behind  it  and 
rendered  it  impervious  to  water.  To»duplicate  this  operation 
by  mechanical  means  on  a  large  enough  scale  to  make  it  ap- 
plicable to  the  construction  of  tunnels  was  the  plan  which 
occurred  to  the  engineer ;  and  how  closely  he  ,f  Allowed  his  ani- 
mate model  may  be  seen  by  examining  tht>  drawings  of  his 
first  shield,  for  which  he  secured  a  patent  in  1818.  Briefly 
described,  this  device  consisted  of  an  iron  cylinder  having  at 
its  front  end  an  auger-like  cutter,  whose  revolution  was  in- 
tended to  shove  away  the  material  ahead  and  thus  advance  the 
cylinder.  As  the  cylinder  advanced  the  perimeter  of  the  hole 
behind  was  to  be  lined  with  a  spiral  sheet-iron  plating,  which 
was  to  be  strengthened  with  an  interior  lining  of  masonry.  It 
will  be  seen  that  the  mechanical  resemblance  of  this  device  to 


SUBMARINE   TUNNELING  243 

the  ship-worm,  on  which  it  is  alleged  to  have  been  modeled,  was 
remarkably  close. 

In  the  same  patent  in  which  Sir  Isambard  secured  protection 
for  his  mechanical  ship-worm  he  claimed  equal  rights  of  inven- 
tion for  another  shield,  which  is  of  far  greater  importance  in 
being  the  prototype  of  the  shield  actually  employed  by  him  in 
constructing  the  first  Thames  tunnel.  This  alternative  inven- 
tion, if  it  may  be  so  termed,  consisted  of  a  group  of  separate 
cells  which  could  be  advanced  one  or  more  at  a  time  or  all 
together.  The  sides  of  these  cells  were  to  be  provided  with 
friction  rollers  to  enable  them  to  slide  easily  upon  each  other ; 
and  it  was  also  specified  that  the  preferable  motive  power  for 
advancing  the  cells  was  hydraulic  jacks.  To  summarize  briefly, 
therefore,  the  two  inventions  of  Brunei  comprehended  the  pro- 
tecting cylinder  or  shield,  the  closure  of  the  face  of  the  exca- 
vation, the  cellular  division,  the  hydraulic-jack  propelling  power, 
and  cylindrical  iron  lining,  which  are  the  essential  characteris- 
tics of  the  modern  shield  system  of  tunneling.  The  next  step 
required  was  the  actual  proof  of  the  practicability  of  Brunei's 
inventions,  and  this  soon  came. 

Those  who  have  read  the  history  of  the  first  Thames  tunnel 
will  recall  the  early  unsuccessful  attempts  at  construction  which 
had  discouraged  English  engineers.  Five  years  after  Brunei's 
patent  was  secured  a  company  was  formed  to  undertake  the 
task  again,  the  plan  being  to  use  the  shield  system,  under  the 
personal  direct*  *^  of  its  inventor  as  chief  engineer.  For  this 
work  Brunei  selected  the  cellular  shield  mentioned  as  an  alter- 
native construction  in  his  original  patent.  He  also  chose  to 
make  this  shield  rectangular  in  form.  This  choice  is  commonly 
accounted  for  by  the  fact  that  the  strata  to  be  penetrated  by  the 
tunnel  were  practically  horizontal,  and  that  it  was  assumed  by 
the  engineer  that  a  rectangular  shield  would  for  spme  reason 
best  resist  the  pressures  which  would  be  developed.  Whatever 
the  reason  may  have  been  for  the  choice,  the  fact  remains  that 
a  rectangular  shield  was  adopted.  The  tunnel  as  designed  con- 


244  TUNNELING 

sisted  of  two  parallel  horseshoe  tunnels,  18ft.  9  ins.  wide  and 
16  ft.  4  ins.  high  and  1200  ft.  long,  separated  from  each  other 
by  a  wall  4  ft.  thick,  pierced  by  64  arched  openings  of  4  ft. 
span,  the  whole  being  surrounded  with  massive  brickwork  built 
to  a  rectangular  section  measuring  over  all  38  ft.  wide  and 
22ft.  high. 

The  first  shield  designed  by  Brunei  for  the  work  proved  in- 
adequate to  resist  the  pressures,  and  it  was  replaced  by  another 
somewhat  larger  shield  of  substantially  the  same  design,  but  of 
improved  construction.  This  last  shield  was  22  ft.  3  ins.  high 
and  37  ft.  6  ins.  wide.  It  was  divided  vertically  into  twelve 
separate  cast-iron  frames  placed  close  side  by  side,  and  each 
frame  was  divided  horizontally  into  three  cells  -capable  of  sepa- 
rate movement,  but  connected  by  a  peculiar  articulated  con- 
struction, which  is  indicated  in  a  general  \fay  by  Fig.  122.  To 
close  or  cover  the  face  of  the  excavation,  poling-boards  held  in 
place  by  numerous  small  screw-jacks  were  employed.  Each 
cell  or  each  frame  could  be  advanced  independently  of  the 
others,  the  power  for  this  operation  being  obtained  by  means 
of  screw-jacks  abutting  against  the  completed  masonry  lining. 
Briefly  described,  the  mode  of  procedure  was  to  remove  the 
poling-boards  in  front  of  the  top  cell  of  one  frame,  and  excavate 
the  material  ahead  for  about  6  ins.  This  being  done,  the  top 
cell  was  advanced  6  ins.  by  means  of  the  screw-jacks,  and  the 
poling-boards  were  replaced.  The  middle  cell  of  the  frame  was 
then  advanced  6  ins.  by  repeating  the  same  process,  and  finally 
the  operation  was  duplicated  for  the  bottom  cell.  With  the 
advance  of  the  bottom  cell  one  frame  had  been  pushed  ahead 
6  ins.,  and  by  a  succession  of  such  operations  the  other  eleven 
frames  were  advanced  a  distance  of  6  ins.,  one  after  the  other, 
until  the  whole  shield  occupied  a  position  6  ins.  in  advance  of 
that  at  which  work  was  begun.  The  next  step  was  to  fill  the 
6-in.  space  behind  the  shield  with  a  ring  of  brickwork. 

The  illustration,  Fig.  122,  is  the  section  parallel  to  the  ver- 
tical plane  of  the  tunnel  through,  the  center  of  one  of  the 


SUBMARINE   TUNNELING 


245 


frames,  and  it  shows  quite  clearly  the  complicated  details  of 
the  shield  construction.  Two  features  which  are  to  be  particu- 
larly noted  are  the  suspended  staging  and  centering  for  con- 


Fio.  122.  -  Longitudinal  Section  of  Brunei's  Shield,  First  Thames  Tunnel. 

structing  the  roof  arch,  and  the  top  plate  of  the  shield  extending 
back  and  overlapping  the  roof  masonry  so  as  to  close  completely 
the  roof  of  the  excavation  and  prevent  it  falling.  Notwithstand- 
ing its  complicated  construction  and  unwieldy  weight  of  120 


246 


TUNNELING 


tons,  this  shield  worked  successfully,  and  during  several  months 
the  construction  proceeded  at  the  rate  of  2  ft.  every  24  hours. 
There  were  two  irruptions  of  water  and  inud  from  the  river 
during  the  work,  but  the  apertures  were  effectually  stopped  by 
heaving  bags  of  clay  into  the  holes  in  the  river  bed,  and  cover- 
ing them  over  with  tarpauling,  with  a  layer  of  gravel  over  all. 
The  tunnel  was  completed  in  1843,  at  a  cost  of  about  $5600 
per  lineal  yard,  and  20  years  from  the  time  work  was  first 
commenced,  including  all  delays. 

The  next  tunnel  to  be  built  by  the  shield  system  was  the 
tunnel  under  London  Tower  constructed  by  Barlow  and 
Greathead  and  begun  in  1869.  In  1863  Mr.  Peter  W.  Barlow 

secured  a  patent  in  England 
for  a  system  of  tunnel  con- 
struction comprising  the  use  of 
a  circular  shield  and  a  cylindri- 
cal cast-iron  lining.  The  shield, 
as  shown  by  Fig.  123,  was 
simply  an  iron  or  steel  plate 
cylinder.  The  cylinder  plates 
were  thinned  down  in  front  to 
form  a  cutting  edge,  and  they 
extended  far  enough  back  at  the  rear  to  enable  the  advance 
ring  of  the  cast-iron  lining  to  be  set  up  within  the  cylinder.  In 
simplicity  of  form  this  shield  was  much  superior  to  Brunei's  ; 
but  it  seems  very  doubtful,  since  it  had  no  diametrical  bracing 
of  any  sort,  whether  it  would  ever  have  withstood  the  com- 
bined pressure  of  the  screw-jacks  and  of  the  surrounding  earth 
in  actual  operation  without  serious  distortion,  and,  probably, 
total  collapse.  It  should  also  be  noted  that  Barlow's  shield 
made  no  provision  for  protecting  the  face  of  the  excavation, 
although  the  inventor  did  state  that  if  the  soil  made  it  neces- 
sary such  a  protection  could  be  used.  The  patent  provided 
for  the  injection  of  liquid  cement  behind  the  cast-iron  lining 
to  fill  the  annular  space  left  by  the  advancing  tail-plates  of  the 


FIG.  123.— First  Shield  Invented  by  Barlow. 


SUBMARINE   TUNNELING 


247 


shield.  Although  Barlow  made  vigorous  efforts  to  get  his 
shield  used,  it  was  not  until  1868  that  an  opportunity  pre- 
sented itself.  In  the  meantime  the  inventor  had  been  studying 
how  to  improve  his  original  device,  and  in  1868  he  secured  addi- 
tional patents  covering  these  improvements.  Briefly  described, 
they  consisted  in  partly  closing  the  shield  with  a  diaphragm, 
as  shown  by  Fig.  124.  The  uninclosed  portion  of  the  shield  is 
here  shown  at  the  center,  but  the  patent  specified  that  it  might 
also  be  located  below  the  center  in  the  bottom  part  of  the 
shield.  The  idea  of  the  construction  was  that  in  case  of  an 
irruption  of  water  the  upper  portion  of  the  shield  could  be 
kept  open  by  air  pressure,  and  work  prosecuted  in  this  open 
space  until  the  shield 
had  been  driven  ahead 
sufficiently  to  close 
the  aperture,  when 
the  normal  condition 
of  affairs  would  be 
resumed.  This  was 
obviously  an  improve- 
ment of  real  merit. 
The  partial  diaphragm 
also  served  to  stiffen  the  shield  somewhat  against  collapse,  but 
the  thin  plate  cutting-edges  and  most  of  the  other  structural 
weaknesses  were  left  unaltered.  To  summarize  briefly  the 
improvements  due  to  Balow's  work,  we  have  :  the  construction 
of  the  shield  in  a  single  piece ;  the  use  of  compressed  air  and 
a  partial  diaphragm  for  keeping  the  upper  part  of  the  shield 
open  in  case  of  irruptions  of  water ;  and  the  injection  of  liquid 
cement  to  fill  the  voids  behind  the  lining. 

Turning  now  to  the  London  Tower  tunnel  work,  it  may 
first  be  noted  that  Barlow  found  some  difficulty  in  finding  a 
contractor  who  was  willing  to  undertake  the  job,  so  little 
confidence  had  engineers  generally  in  his  shield  system.  One 
man,  however,  Mr.  J.  H.  Greathead,  perceived  that  Barlow's 


Longitudinal  Section..  Cross  Section . 

FIG.  124.  —  Second  Shield  Invented  by  Barlow. 


248  TUNNELING 

device  presented  merit,  although  its  design  and  construction 
were  defective,  and  he  finally  undertook  the  work  and  carried 
it  to  a  brilliant  success.  The  tunnel  was  1,350  ft.  long  and 
7  ft.  in  diameter,  and  penetrated  compact  clay.  Work  was 
begun  on  the  first  shore  shaft  on  Feb.  12,  1869,  and  the  tunnel 
was  completed  the  following  Christmas,  or  in  something  short 
of  eleven  months,  at  a  cost  of  c£  14,5  00. 

The  shield  used  was  Barlow's  idea  put  into  practical  shape 
by  Greathead.  It  consisted  of  an  iron  cylinder,  or,  more 
properly,  a  frustum  of  a  cone  whose  circumferential  sides 
were  very  slightly  inclined  to  the  axis,  the  idea  being  that 
the  friction  would  be  less  if  the  front  end  of  the  shield  were 
slightly  larger  than  the  rear  end.  The  shell  of  the  cone  was 
made  of  J  in.  plates.  The  thinned  plate  cutting-edge  of 
Barlow's  shield  was  replaced  by  Greathead  with  a  circular 
ring  of  cast  iron.  Greathead  also  altered  the  construction  of 
the  diaphragm  by  arranging  the  angle  stiffeners  so  that  they 
ran  horizontally  and  vertically,  and  by  fastening  the  diaphragm 
plates  to  an  interior  cast-iron  ring  connected  to  the  shell  plates. 
This  was  a  decided  structural  improvement,  but  it  was  accom- 
panied with  another  modification  which  was  quite  as  decided 
a  retrogression  from  Barlow's  design.  Greathead  made  the 
diaphragm  opening  rectangular  and  to  extend  very  nearly  from 
the  top  to  the  bottom  of  the  shield,  thus  abandoning  the 
element  of  safety  provided  by  Barlow  in  case  of  an  irruption 
of  water.  Fortunately  the  material  penetrated  by  the  shield 
for  the  Tower  tunnel  was  so  compact  that  no  trouble  was  had 
from  water ;  but  the  dangerous  character  of  the  construction 
was  some  years  afterwards  disastrously  proven  in  driving  the 
Yarra  River  tunnel  at  Melbourne,  Australia.  To  drive  his 
shield  Greathead  employed  six  2^  in.  screw-jacks  capable  of 
developing  a  total  force  of  60  tons.  The  tails  of  the  jack  bore 
against  the  completed  lining,  which  consisted  of  cast-iron  rings 
18  ins.  wide  and  £  in.  thick,  each  ring  being  made  up  of  a 
crown  piece  and  three  segments.  The  different  segments 


SUBMARINE    TUNNELING 


249 


and  rings  were  provided  with  double  (exterior  and  interior) 
flanges,  by  means  of  which  they  were  bolted  together.  The 
soil  behind  the  lining  was  filled  with  liquid  cement  injected 
through  small  holes  by 
means  of  a  hand  pump. 
The  remarkable  suc- 
cess of  the  London  Tower 
tunnel  encouraged 
Barlow  to  form  in  1871  a 
company  to  tunnel  the 

Thames     between    South-      FlG-  125«  —  Shield  Suggested  by  Greathead  for  the 

Proposed  North  and  South  Woolwich  Subway. 

wark  and  the  City,  and 

Greathead,  in  1876,  to  project  a  tunnel  under  the  same  water- 
way known  as  the  North  and  South  Woolwich  Subway.  Bar- 
low's concession  was  abrogated  by  Parliament  in  1873,  without 

any  work  having  been 
done.  Greathead  pro- 
gressed far  enough  with 
his  enterprise  to  construct 
a  shield  and  a  large 
amount  of  the  iron  lining 
when  the  contractors 
abandoned  the  work. 
From  the  brief  descrip- 
tion of  his  shield  given 
by  Greathead  to  the  Lon-^ 
don  Society  of  Civil  En- 
gineers, it  contained  sev- 
eral important  differences 
from  the  shield  built  by 
him  for  the  London 
Tower  tunnel,  as  is  shown 
by  Fig.  125.  The  changes 

which  deserve  particular  notice  are  the  great  extension  of  the 
shield  behind  the  diaphragm,  the  curved  form  of  the  diaphragm, 


PIG. 


126— Beach's  Shield   Used   on  Broadway 
Pneumatic  Railway  Tunnel. 


250 


TUNNELING 


and  the  use  of  hydraulic  jacks.  Greathead  had  also  designed 
for  this  work  a  special  crane  to  be  used  in  erecting  the  cast-iron 
segments  of  the  lining. 

While  these  works  had  been  progressing  in  England,  Mr. 
Beach,  an  American,  received  a  patent  in  the  United  States  for 
a  tunnel  shield  of  the  construction  shown  by  Fig.  126,  which 
was  first  tried  practically  in  constructing  a  short  length  of 
tunnel  under  Broadway  for  the  nearly  forgotten  Broadway 


FIG.  127.  —  Shield  for  City  and  South  London  Railway. 

Pneumatic  Underground  Ry.  This  shield,  as  is  indicated  by 
the  illustration,  consisted  of  a  cylinder  of  wood  with  an  iron- 
cutting-edge  and  an  iron  tail-ring.  Extending  transversely 
across  the  shield  at  the  front  end  were  a  number  of  horizontal 
iron  plates  or  shelves  with  cutting-edges,  as  shown  clearly  by 
the  drawing.  The  shield  was  moved  ahead  by  means  of  a 
number  of  hydraulic  jacks  supplied  with  power  by  a  hand 
pump  attached  to  the  shield.  By  means  of  suitable  valves  all 
or  any  lesser  number  of  these  jacks  could  be  operated,  and  by 


S?  0< 

d* 


SUBMARINE   TUXXELIXG 


251 


aJA-MsVVW^V; 


•ft!       t       T    i  iSaJKT.t 
"i  •       -      tit: 


V        nkrJ?i'61/i/ 
^•HolKfbr^bohs^ 


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e.o_£_o..o_''_e.£. 


...,_       SECTION  SHOWING  HALF  OF  VV\LL  F.  ~ -j> ^SECTION  SHOWING  HALF  OF  WALL  E, 


252 


TUNNELING 


thus  regulating  the  action  of  the  motive  power  the  direction  of 
the  shield  could  be  altered  at  will.  Work  was  abandoned  on 
the  Broadway  tunnel  in  1870.  In  1871-2  Beach's  shield  was 
used  in  building  a  short  circular  tunnel  8  ft.  in  diameter  in 
Cincinnati,  and  a  little  later  it  was  introduced  into  the  Cleve- 
land water-works  tunnel  8  ft.  in  diameter.  In  this  latter  work, 
which  was  through  a  very  treacherous  soil,  the  shield  gave  a 
great  deal  of  trouble,  and  was  finally  so  flattened  by  the 


Longitudinal  Section,  Cross  Section. 

Fm.  129.  — Shield  for  BlackAvall  Tunnel. 

pressures  that  it  was  abandoned.  The  obviously  defective  fea- 
tures of  this  shield  were  its  want  of  vertical  bracing  and  the 
lack  of  any  means  of  closing  the  front  in  soft  soil. 

With  the  foregoing  brief  review  of  the  early  development  of 
the  shield  system  of  tunneling,  we  have  arrived  at  a  point  where 
the  methods  of  modern  practice  can  be  studied  intelligently. 
In  the  pages  which  follow  we  shall  first  illustrate  fully  the 
construction  of  a  number  of  shields  of  typical  and  special 
construction,  and  follow  these  illustrations  with  a  general  dis- 
cussion of  present  practice  in  the  various  details  of  shield 
construction. 


SUBMARINE   TUNNELING 


253 


Transverse     Section. 


Longitudinal  Section. 

FIG.  130.  —  Elliptical  Shield  for  Clichy  Sewer  Tunnel,  Paris. 

Mr.  Raynald  Legouez,  in  his  excellent  book  upon  the  shield 
system  of  tunneling,  considers  that  tunnel  shields  may  be  di- 
vided into  three  classes  structurally,  according  to  the  character 


254 


TUNNELING 


of  the  material  which  they  are  designed  to  penetrate.  In  the 
first  class  he  places  shields  designed  to  work  in  a  stiff  and  com- 
paratively stable  soil,  like  the  well-known  London  clay ;  in  the 
second  class  are  placed  those  constructed  to  work  in  soft  clays 
r  nd  silts ;  and  in  the  third  class  those  intended  for  soils  of  an 


Longitudinal  Section. 


Cross    Section. 

FIG.  131.  — Semi  Elliptical  Shield  for  Clichy  Sewer  Tunnel. 

unstable  granular  nature.  This  classification  will,  in  a  general 
way,  be  kept  by  the  writer.  As  a  representative  shield  of 
the  first  class,  the  one  designed  for  the  City  and  South  London 
Ry.  is  illustrated  in  Fig.  127.  The  shields  for  the  London 
Tower  tunnel,  the  Waterloo  &  City  Ry.,  the  Glasgow  District 
Subway,  the  Siphons  of  Cliehy  and  Concorde  in  Paris,  and  the 


SUBMARINE   TUNNELING 


255 


Glasgow  Port  tunnel,  are  of  the  same  general  design  and  con- 
struction. To  represent  shields  of  the  second  class,  the  St. 
Clair  River  and  Black  wall  shields  are  shown  in  Figs.  128  and 
129.  The  shields  for  the  Mersey  River,  the  Hudson  River, 
and  the  East  River  tunnels  also  belong  to  this  class.  To 
represent  shields  of  the  third  class,  the  elliptical  and  semi- 

> ,(f , ;..__.. *V  . 


Details  of  Casting 
Supporting  Ends  of  Jack*. 


Details  of  Casting  under  Ends 
of  Girders. 


Longitudinal    Section  C-Q. 

FIG.  132.  —  Roof  Shield  for  Boston  Subway. 

elliptical  shields  of  the  Clichy  tunnel  work  in  Paris  are  shown 
by  Figs.  130  and  131.  The  semi-circular  shield  of  the  Boston 
Subway  is  illustrated  by  Fig.  132. 


SHIELD    CONSTRUCTION. 

General  Form.  —  Tunnel  shields  are  usually  cylindrical  or 
semi-cylindrical  in  cross-section.  The  cylinder  may  be  circular, 
elliptical,  or  oval  in  section.  Far  the  greater  number  of  shields 
used  in  the  past  have  been  circular  cylinders ;  but  in  one  part 
of  the  sewer  tunnel  of  Clichy,  in  Paris,  an  elliptical  shield 


256  TUNNELING 

with  its  major  axis  horizontal,  was  used,  and  the  German  en- 
gineer, Herr  Mackensen,  has  designed  an  oval  shield,  with  its 
major  axis  vertical.  A  semi-elliptical  shield  was  employed  on 
the  Clichy  tunnel,  and  semi-circular  shields  were  used  on  the 
Baltimore  Belt  Line  tunnel  and  the  Boston  Subway  in  Amer- 
ica. Generally,  also,  tunnel  shields  are  right  cylinders ;  that  is, 
the  front  and  rear  edges  are  in  vertical  planes  perpendicular  to 
the  axis  of  the  cylinder.  Occasionally,  however,  they  are 
oblique  cylinders ;  that  is,  the  front  or  rear  edges,  or  both,  are 
in  planes  oblique  to  the  axis  of  the  cylinder.  One  of  these 
visor-shaped  shields  was  employed  on  the  Clichy  tunnel. 

The  Shell,  —  It  is  absolutely  necessary  that  the  exterior  sur- 
face of  the  shell  should  be  smooth,  and  for  this  reason  the 
exterior  rivet  heads  must  be  countersunk.  It  is  generally 
admitted,  also,  that  the  shell  should  be  perfectly  cylindrical, 
and  not  conical.  The  conical  form  has  some  advantage  in 
reducing  the  frictional  resistance  to  the  advance  of  the  shield ; 
but  this  is  generally  considered  to  be  more  than  counterbalanced 
by  the  danger  of  subsidence  of  the  earth,  caused  by  the  exces- 
sive void  which  it  leaves  behind  the  iron  tunnel  lining.  For 
the  same  reason  the  shell  plate,  which  overlaps  the  forward  ring 
of  the  lining,  should  be  as  thin  as  practicable,  but  its  thickness 
should  not  be  reduced  so  that  it  will  deflect  under  the  earth 
pressures  from  above.  Generally  the  shell  is  made  of  at  least 
two  thicknesses  of  plating,  the  plates  being  arranged  so  as  to 
break  joints,  and,  thus,  to  avoid  the  use  of  cover  joints,  to  inter- 
rupt the  smooth  surface  which  is  so  essential,  particularly  on 
the  exterior.  The  thickness  of  the  shell  required  will  vary 
with  the  diameter  of  the  shield,  and  the  character  and  strength 
of  the  diametrical  bracing.  Mr.  Raynald  Legouez  suggests  as 
a  rule  for  determining  the  thickness  of  the  shell,  that,  to  a 
minimum  thickness  of  2  mm.,  should  be  added  1  mm.  for  every 
meter  of  diameter  over  4  meters.  Referring  to  the  illustrations, 
Figs.  128  to  132  inclusive,  it  will  be  noted  that  the  St.  Clair 
tunnel  shield,  21^  ft.  in  diameter,  had  a  shell  of  1-in.  steel 


SUBMARINE    TUNNELING  257 

plates  with  cover-plate  joints  and  interior  angle  stiffeners  ;  the 
shell  of  the  East  River  tunnel  shield,  11  ft.  in  diameter,  was 
made  up  of  one  £-in.  and  one  f-in.  plate;  the  Blackwall  tunnel 
shield,  27  ft.  9  ins.  in  diameter,  had  a  shell  consisting  of 
four  thicknesses  of  f-in.  plates ;  and  the  Clichy  tunnel  shield, 
with  a  diameter  of  2.06  meters,  had  a  shell  2  millimeters  thick. 
Front-End  Construction.  —  By  the  front  end  is  meant  that 
portion  of  the  shield  between  the  cutting-edge  and  the  vertical 
diaphragm.  The  length  of  this  portion  of  the  shield  was 
formerly  made  quite  small,  and  where  the  material  penetrated 
is  very  soft,  a  short  front-end  construction  yet  has  many  advo- 
cates ;  but  the  general  tendency  now  is  to  extend  the  cutting- 
edge  far  enough  ahead  of  the  diaphragm  to  form  a  fair-sized 
working  chamber.  Excavation  is  far  more  easy  and  rapid  when 
the  face  can  be  attacked  directly  from  in  front  of  the  diaphragm 
than  where  the  work  has  to  be  done  form  behind  through  the 
apertures  in  the  diaphragm.  So  long  as  the  roof  of  the  excava- 
tion is  supported  from  falling,  experience  has  shown  that  it  is 
easily  possible  to  extend  the  excavation  safely  some  distance 
ahead  of  the  diaphragm.  In  reasonably  stable  material,  tike 
compact  clay,  the  front  face  will  usually  stand  alone  for  the 
short  time  necessary  to  excavate  the  section  and  advance  the 
shield  one  stage.  In  softer  material  the  face  can  usually  be 
sustained  for  the  same  short  period  by  means  of  compressed  air ; 
or  the  face  of  the  excavation,  instead  of  being  made  vertical,  can 
be  allowed  to  assume  its  natural  slope.  In  the  latter  case  a 
visor-shaped  front-end  construction,  such  as  was  used  on  some 
portions  of  the  Clichy  tunnel,  is  particularly  advantageous.  The 
following  figures  show  the  lengths  of  the  front  ends  of  a  number 
of  representative  tunnel  shields. 

City  and  South  London     .          1  ft.  Mersey  River 3  ft. 

St.  Clair  River     ....  11.25   "  East  River 31  " 

Hudson  River      ....        53   "  Blackwall 6.5    " 

Two  general  types  of  construction   are   employed  for  the 
cutting-edge.     The  first  type  consists  of  a  cast-iron  or  cast-steel 


258 


TUNNELING 


ring,  beveled  to  form  a  chisel-like  cutting-edge,  and  bolted  to 
the  ends  of  the  forward  shell  plates.  This  construction  was 
first  employed  in  the  shield  for  the  London  Tower  tunnel,  and 
has  since  been  used  on  the  City  and  South  London,  Waterloo 
and  City,  and  the  Clichy  tunnels.  The  second  construction 
consists  in  bracing  the  forward  shell  plates  by  means  of  right 
triangular  brackets,  whose  perpendicular  sides  are  riveted 
respectively  to  the  shell  plates  and  the  diaphragm,  and  whose 
inclined  sides  slant  backward  and  downward  from  the  front 
edge,  and  carry  a  conical  ring  of  plating.  The  shields  for  the 
St.  Clair  River,  East  River,  and  Blackwall  tunnels  show  forms 
of  this  type  of  cutting-edge  construction.  A  modification  of 
the  second  type  of  construction,  which  consists  in  omitting  the 
conical  plating,  was  employed  on  some  of  the  shields  for  the 
Clichy  tunnel.  This  modification  is  generally  considered  to  be 
allowable  only  in  materials  which  have  little  stability,  and  which 
crumble  down  before  the  advance  of  the  cutting-edge.  Where 
the  material  is  of  a  sticky  or  compact  nature,  into  which  the 
shield  in  advancing  must  actually  cut,  the  beveled  plating  is 
necessary  to  insure  a  clean  cutting  action  without  wedging  or 
jamming  of  the  material. 

Cellular  Division.  —  It  is  necessary  in  shields  of  large  diam- 
eter to  brace  the  shell  horizontally  and  vertically  against 
distortion.  This  bracing  also  serves  to  form  stagings  for  the 
workmen,  and  to  divide  the  shield  into  cells.  The  following 
table  shows  the  arrangement  of  the  vertical  and  transverse 
bracing  in  several  representative  tunnel  shields. 


NAME  OF  TUNNEL. 

DlAMETEB. 

HORI- 
ZONTAL. 

PLATES, 

DlST. 

APAKT. 

VERT. 
BRACES. 

Hudson  River      

Ft. 
19 
19.4 
21 
24 
27 
11 

In. 
11 
0 
6 
10* 
8 
1 

No. 
2 
2 
2 
2 
2 
None 

Ft. 
6.54 
6.54 
6.98 
7.12 
6.0 

No. 
2 
None 
3 
None 
3 
1 

Clichy    

St.  Clair  River     

Waterloo  (Station)  .... 
Blackwall 

East  River 

STJBMAKINE   TUNNELING  259 

Referring  first  to  the  horizontal  divisions,  it  may  be  noted 
that  they  serve  different  purposes  in  different  instances.  In  the 
Clichy  tunnel  shield  the  horizontal  divisions  formed  simply 
working  platforms  ,  in  the  Waterloo  tunnel  shield  they  were 
designed  to  abut  closely  against  the  working  face  by  means  of 
special  jacks,  and  so  to  divide  it  into  three  separate  divisions ;  in 
the  St.  Glair  tunnel  they  served  as  working  platforms,  and  also 
had  cutting-edges  for  penetrating  the  material  ahead;  and  in 
the  Blackwall  tunnel  shield  they  served  as  working  platforms, 
and  had  cutting-edges  as  in  the  St.  Clair  tunnel  shield,  and  in 
addition  the  middle  division  was  so  devised  that  the  two  lower 
chambers  of  the  shield  could  be  kept  under  a  higher  pressure  of 
air  than  the  two  upper  chambers.  Passing  now  to  the  vertical 
divisions,  they  serve  to  brace  the  shell  of  the  shield  against  ver- 
tical pressures,  and  also  to  divide  the  horizontal  chambers  into 
cells;  but  unlike  the  horizontal  plates  they  are  not  provided 
with  cutting-edges.  The  St.  Clair,  Hudson  River,  and  Black- 
wall  tunnel  shields  illustrate  the  use  of  the  vertical  bracirg  for 
the  double  purpose  of  vertical  bracing  and  of  dividing  the  hori- 
zontal chambers  into  cells.  The  Waterloo  tunnel  shield  is 
an  example  of  vertical  bracing  employed  solely  as  bracing. 
The  vertical  division  of  the  East  River  tunnel  shield  was 
employed  in  order  to  allow  the  shield  to  be  dissembled  in 
quadrants. 

The  Diaphragm.  —  The  purpose  of  the  shield  diaphragm  is  to 
close  the  rear  end  of  the  shield  and  the  tunnel  behind  from  an 
inrush  of  water  and  earth  from  the  face  of  the  excavation.  It 
also  serves  the  secondary  purpose  of  stiffening  the  shell  diamet- 
rically. Structurally  the  diaphragm  separates  the  front-end  con- 
struction previously  described  from  the  rear-end  construction, 
which  will  be  described  farther  on ;  and  it  is  usually  composed 
of  iron  or  steel  plating  reinforced  by  beams  or  girders,  and 
pierced  with  one  or  several  openings  by  which  access  is  had 
to  the  working  face.  In  stable  material,  where  caving  or  an 
inrush  of  water  and  earth  is  not  likely,  the  diaphragm  is 


260  TUNNELING 

omitted.  The  shield  of  the  Waterloo  tunnel  is  an  example  of 
this  construction.  In  more  treacherous  materials,  however,  not 
only  is  a  diaphragm  necessary,  but  it  is  also  necessary  to  diminish 
the  size  of  the  openings  through  it,  and  to  provide  means  for 
closing  them  entirely.  Sometimes  only  one  or  two  openings  are 
left  near  the  bottom  of  the  diaphragm,  as  in  the  St.  Clair  and 
Mersey  tunnel  shields ;  and  sometimes  a  number  of  smaller 
openings  are  provided,  as  in  the  East  River  and  Hudson  River 
tunnel  shields. 

In  highly  treacherous  materials  subject  to  sudden  and 
violent  irruptions  of  earth  from  the  excavation  face,  it  some- 
times is  the  case  that  openings,  however  small,  closed  in  the 
ordinary  manner,  are  impracticable,  and  special  construction  .has 
to  be  adopted  to  deal  with  the  difficulty.  The  shields  for  the 
Mersey  and  for  the  Blackwall  tunnels  are  examples  of  such 
special  devices.  In  the  Mersey  tunnel  a  second  diaphragm  was 
built  behind  the  first,  extending  from  the  bottom  of  the  shield 
upward  to  about  half  its  total  height.  The  aperture  in  the  first 
diaphragm  being  near  the  bottom,  the  space  between  the  second 
and  first  diaphragms  f  ormed  a  trap  to  hold  the  inflowing  material. 
The  Blackwall  tunnel  shield,  as  previously  indicated,  had  its 
front  end  divided  into  cells.  Ordinarily  the  face  of  the  excava- 
tion in  front  of  each  cell  was  left  open,  but  where  material  was 
encountered  which  irrupted  into  these  cells  a  special  means  of 
closing  the  face  was  necessary.  This  consisted  of  three  poling- 
boards  or  shutters  of  iron  held  one  above  the  other  against  the 
face  of  the  excavation.  These  shutters  were  supported  by 
means  of  strong  threaded  rods  passing  through  nuts  fastened 
to  the  vertical  frames,  which  permitted  each  shutter  to  be  ad- 
vanced against  or  withdrawn  from  the  face  of  the  excavation 
independently  of  the  others.  Various  other  constructions  have 
been  devised  to  retain  the  face  of  the  excavation  in  highly 
treacherous  soils,  but  few  of  them  have  been  subjected  to 
conclusive  tests,  and  they  do  not  therefore  justify  considera- 
tion. 


SUBMARINE    TUNNELING  261 

Rear-End  Construction.  —  By  the  rear  end  of  the  shield  is 
meant  that  portion  at  the  rear  of  the  diaphragm.  It  may  be 
divided  into  two  parts,  called  respectively  the  body  and  the 
tail  of  the  shield.  The  chief  purpose  of  the  body  of  the  shield 
is  to  furnish  a  place  for  the  location  of  the  jacks,  pumps, 
motors,  etc.,  employed  in  manipulating  the  shield.  It  also 
serves  a  purpose  in  distributing  the  weight  of  the  shield  over 
a  large  area.  To  facilitate  the  passage  of  the  shield  around 
curves,  or  in  changing  from  one  grade  to  another,  it  is  desirable 
to  make  the  body  of  the  shield  as  short  as  possible.  In  the 
Mersey,  Clichy,  and  Waterloo  tunnel  shields,  and,  in  fact, 
in  most  others  which  have  been  employed,  the  shell  plates  of 
the  body  have  been  reinforced  by  a  heavy  cast-iron  ring,  within 
and  to  which  are  attached  the  jacks  and  other  apparatus.  The 
latest  opinion,  however,  seems  to  point  to  the  use  of  brackets 
and  beams  for  strengthening  the  shell  for  the  purpose  named, 
rather  than  to  this  heavy  cast-iron  construction.  In  the 
Hudson  River,  St.  Clair  River,  and  East  River  tunnel  shields, 
with  their  long  and  strongly  braced  front-end  construction  to 
carry  the  jacks,  the  body  of  the  shield,  so  to  speak,  is  omitted, 
and  the -rear-end  construction  consists  simply  of  the  tail  plat- 
ing. In  the  Black  wall  shield,  the  body  of  the  shield  shell 
provides  the  space  necessary  for  the  double  diaphragms  and 
the  cells  which  they  inclose.  In  a  general  way,  it  may  be 
said  that  the  present  tendency  of  engineers  is  to  favor  as 
short  and  as  light  a  body  construction  as  can  be  secured. 

The  tail  of  the  shield  serves  to  support  the  earth  while  the 
lining  is  being  erected;  and  for  this  reason  it  overlaps  the 
forward  ring  of  the  lining,  as  shown  clearly  by  most  of 
the  shields  illustrated.  To  fulfill  this  purpose,  the  tail-plates 
should  be  perfectly  smooth  inside  and  outside,  so  as  to  slide 
easily  between  the  outside  of  the  lining  plates  and  the  earth, 
and  should  also  be  as  thin  as  practicable,  in  order  not  to  leave 
a  large  void  behind  the  lining  to  be  filled  in.  In  soils  which 
are  fairly  stable,  the  tail  construction  is  often  visor-shaped : 


262 


TUNNELING 


that  is,  the  tail-plates  overlap  the  lining  only  for,  say,  the  roof 
from  the  springing  lines  up,  as  in  one  of  the  shields  for  the 
Clichy  tunnel.  In  unstable  materials,  the  tail-plating  ex- 
tends entirely  around  the  shield  and  excavation.  The  length 
of  the  tail-plating  is  usually  sufficient  to  overlap  two  rings  of 
the  lining,  but  in  one  of  the  Clichy  tunnel  shields  it  will  be 
noticed  that  it  extended  over  three  rings  of  lining.  This 
seemingly  considerable  space  for  thin  steel  plates  is  made 
possible  by  the  fact  that  the  extreme  rear  end  of  the  tail 
always  rests  upon  the  last  completed  ring  of  lining. 

In  closing  these  remarks  concerning  the  rear-end  con- 
struction, the  accompanying  table,  prepared  by  Mr.  Raynald 
Legouez,  will  be  of  interest,  as  a  general  summary  of  principal 
dimensions  of  most  of  the  important  tunnel  shields  which  have 
been  built.  The  figures  in  this  table  have  been  converted 
from  metric  to  English  measure,  and  some  slight  variation. 
from  the  exact  dimensions  necessarily  exists.  The  different 
columns  of  the  table  show  the  diameter,  total  length,  and  the 
length  of  each  of  the  three  principal  parts  into  which  tunnel 
shields  are  ordinarily  divided  in  construction  as  previously 
described :  — 


NAME  OF  SHIKLD. 

LENGTH  IN  FEET. 

DIAMETER. 

TAIL. 

BODY. 

FRONT. 

TOTAL. 

Concorde  Siphon  .... 

6  75 

2.51 

2.55 

1.16 

6.67 

Clichy  Siphon  

8.39 

2.51 

2.55 

1.16 

6.16 

Mersey     

9.97 

5.61 

2.98 

2.98 

11.58 

East  River    

10.99 

3.51 

0.32 

3.67 

7.51 

City  and  South  London 

10.99 

2.65 

2.82 

1  01 

641) 

Glasgow  District        .     .     . 

12.07 

2.65 

2.82 

1.01 

6.49 

Waterloo  and  City     .     .     . 

12.99 

2.75 

2.98 

1.24 

6.T8 

Glasgow  Harbor    .... 

17.25 

2.75 

2.98 

1.08 

8.49 

Hudson  River  

19.91 

4.82 

2.1)8 

5.67 

10.49 

St   Clair  River 

21  52 

4  00 

2  98 

11  25 

1")  25 

Clichy  Tunnel 

237  198 

4  00 

2  98 

6  88 

17  °2 

Clichy  Tunnel 

23  8-19  4 

7  44 

11  90 

4  46 

23  6") 

Blackwall     .          .... 

27  t 

6  98 

5  90 

6  59 

19  48 

Waterloo  Station  .... 

24.86 

3.34 

5.51 

1.14 

10.00 

SUBMARINE    TUNNELING  263 

Jacks The    motive   power  usually  employed    in   driving 

modern   tunnel    shields  is  hydraulic   jacks.     In    some   of    the 
earlier  shields  screw-jacks  were  used,  but  these  soon  gave  way 
to    the    more    powerful   hydraulic    device.       The    manner    of 
attaching  the  hydraulic  jacks  to  the  shield  is  always  to  fasten 
the  cylinder  castings  at  regular  intervals  around  the  inside   of 
the  shell,  with  the  piston  rods  extending  backward  to  a  bearing 
against  the  forward  edge  of  the  lining.     In  the  older  forms  of 
shield,  having  an  interior  cast>iron  reinforcing  ring  construc- 
tion, the  jack  cylinder  castings  were  always  attached  to  this 
cast-iron  ring;    but  in  many  of  the   later  shields  constructed 
without  this   cast-iron  reinforcing  ring,   the  cylinder  castings 
are  attached  to  the  shell  by  means  of  bracket  and  gusset  con- 
nections.    The  number  and  size  of  the  jacks  employed,  and  the 
distance  apart  at  which  they  are  spaced,  depend  upon  the  size 
of  the  shield  and  the   character  of  the  material  in  which  it  is 
designed  to  work.     In  stiff  and  comparatively  stable  clays,  the 
skin  friction  of  the  shield  is  comparatively  small,  and  an  ag- 
gregate  jack-power    of   from  4  to  5  tons    per  square  yard  of 
the    exterior   friction    surface   of  the   shield  has  usually  been 
found  ample.     The   cylinders   are   spaced  about  5|  ft.  apart, 
and   have    a   working  diameter  of  from  5    to   6  ins.,  with   a 
water    pressure    of    about    1,000    Ibs.    per    sq.    in.     In    soft, 
sticky  material,  giving  a  high  skin  friction,  the  aggregate  jack- 
power  required  per  square  yard  of  exterior  shell  surface  rises  to 
from  18  to  24  tons;    the  jacks  are  spaced  about  3  ft.  apart; 
and  the  working  cylinder  diameter  and  water  pressure  are,  re- 
spectively, about  6  or  7  ins.,  and  from  4,000  Ibs.  to  6,000  Ibs. 
per   sq.    in.     With   these    high    pressures,    power   pumps    are 
necessary  to  give  the  required  water  pressure ;   but  where  the 
pressure  required  does  not  exceed  1,000  Ibs.  per  sq.  in.,  hand 
pumps  may  be,  and  usually  are,  employed.     The   number  of 
jacks  required  depends  upon  the  diameter  of  the  shield,  and,  of 
course,  upon  the  distance  apart  which  they  are  placed.     In  the 
City  and  South  London  tunnel  shield  six  jacks  were  used,  and 


264 


TUNNELING 


in  the  Blackwall  shield  24  were  used.  The 
construction  of  the  jacks  for  tunnel  shields  presents  no  features 
out  of  the  usual  lines  of  such  devices  used  elsewhere.  The 
jacks  used  on  the  East  River  tunnel  shield  are  shown  by  Fig. 
118,  and  those  for  the  St.  Clair  River  tunnel  by  Fig.  133. 


Part  Transverse  Section.  Longitudinal   Section. 

FIG.  133.  —  Cast  Iron  Lining,  St.  Clair  River  Tunnel. 

Two  general  methods  are  employed  for  transmitting  the 
thrust  of  the  piston  rods  against  the  tunnel  lining.  The 
object  sought  in  each  is  to  distribute  the  thrust  in  such  a 
manner  that  there  is  no  danger  of  bending  the  thin  front  flange 
of  the  forward  lining  ring.  In  English  practice  the  plan 


SUBMARINE    TUNNELING  265 

usually  adopted,  is  to  attach  a  shoe  or  bearing  casting  to  the 
end  of  the  piston  rod,  which  will  distribute  the  pressure  over 
a  considerable  area.  An  example  of  this  construction  is  the 
shield  for  the  City  and  South  London  tunnel.  In  the  East 
River  and  St.  Clair  River  tunnels,  built  in  America,  the  tail  of 
the  piston  rod  is  so  constructed  that  the  thrust  is  carried 
directly  to  the  shell  of  the  lining. 

LINING. 

Either  iron  or  masonry  may  be  used  for  lining  shield-driven 
tunnels  but  present  practice  is  almost  universally  in  favor  of 
iron  lining.  As  usually  built,  iron  lining  consists  of  a  series  of 
successive  cast  iron  rings,  the  abutting  edges  of  which  are  pro- 
vided with  flanges.  These  flanges  are  connected  by  means  of 
butts,  the  joints  being  packed  with  thin  strips  of  wood,  oakum, 
cement,  or  some  other  material  to  make  them  water-tight. 
Each  lining  ring  is  made  up  of  four  or  more  segments,  which 
are  provided  with  flanges  for  bolted  connections  similar  to 
those  fastening  the  successive  rings.  Generally  the  crown  seg- 
ment is  made  considerably  shorter  than  those  forming  the  sides 
and  bottom  of  the  ring.  The  erection  of  the  iron  segments 
forming  the  successive  rings  of  the  lining  may  be  done  by  hand 
in  tunnels  of  small  diameter  where  the  weights  to  be  handled 
are  comparatively  light,  but  in  tunnels  of  large  size  special 
cranes  attached  to  the  shield  or  carried  by  the  finished  lining 
are  employed.  The  construction  of  the  iron  lining  for  the  East 
River  tunnel  is  illustrated  in  Chapter  XIX.,  and  that  for  the 
St.  Clair  River  tunnel  is  shown  by  Fig.  133. 


266  TUNNELING 


CHAPTER  XXII. 

ACCIDENTS  AND  REPAIRS  IN  TUNNELS  DURING 
AND  AFTER  CONSTRUCTION. 


IN  the  excavation  of  tunnels  it  often  happens  that  the  dis- 
turbance of  the  equilibrium  of  the  surrounding  material  by  the 
excavation  develops  forces  of  such  intensity  that  the  timbering 
or  lining  is  crushed  and  the  tunnel  destroyed.  To  provide 
against  accidents  of  this  kind  in  a  theoretically  perfect  manner 
would  require  the  engineer  to  have  an  accurate  knowledge  of 
the  character,  direction  and  intensity  of  the  forces  developed, 
and  this  is  practically  impossible,  since  all  of  these  factors  differ 
with  the  nature  and  structure  of  the  material  penetrated.  The 
best  that  can  be  done,  therefore,  is  to  determine  the  general 
character  and  structure  of  the  material  penetrated,  as  fully  as 
practicable,  by  means  of  borings  and  geological  surveys,  and 
then  to  employ  timbering  and  masonry  of  such  dimensions  and 
character  as  have  withstood  successfully  the  pressures  devel- 
oped in  previous  tunnels  excavated  through  similar  material. 
If,  despite  these  precautions,  accidents  occur,  the  engineer  is 
compelled  to  devise  methods  of  checking  and  repairing  them, 
and  it  is  the  purpose  of  this  chapter  to  point  out  briefly  the 
most  common  kinds  of  accidents,  their  causes,  and  the  usual 
methods  of  repairing  them. 

Accidents  During  Construction.  — Accidents  may  happen  both 
during  or  after  construction,  but  it  is  during  construction,  when 
the  equilibrium  of  the  surrounding  material  is  first  disturbed, 
and  when  the  only  support  of  the  pressures  developed  is  the 
timber  strutting  that  they  most  commonly  occur. 


ACCIDENTS    AND    REPA1KS    IN    TUNNELS  267 

Causes  of  Collapse.  —  Collapse  in  tunnels  may  be  caused :  (1) 
by  the  weight  of  the  earth  overhead,  which  is  left  unsupported 
by  the  excavation ;  (2)  by  defective  or  insufficient  strutting  ; 
and  (3)  by  defective  or  weak  masonry. 

(1)  The  danger  of  collapse  of  the  roof  of  the  excavation  is 
influenced  by  several  conditions.     One  of  these  is  the  method 
of   excavation   adopted.      It   is   obvious   that    the    larger   the 
volume    of    the  supporting    earth    is,  which    is    removed,  the 
greater  will  be  the  tendency  of  the  roof  to  fall,  and  the  more 
intense  will  be  the  pressures  which  the  strutting  will  be  called 
upon  to  support.     Thus  the  English  and   Austrian  methods  of 
tunneling,  where  the  full  section  is  excavated  before  any  of  the 
lining  is  placed,  and  where,  as  the  consequence,  the  strutting 
has  to  sustain  all  of  the  pressures,  piesent  more  likelihood  of 
the  roof  caving  in  than  any  of  the  other  common  methods. 

The  character  and  structure  of  the  material  penetrated  also 
influence  the  danger  of  a  collapse.  A  loose  soil  with  little 
cohesion  is  of  course  more  likely  to  cave  than  one  which  is 
more  stable.  Rock  where  strata  are  horizontal,  or  which  is 
seamy  and  fissured,  is  more  likely  to  break  down  under  the  roof 
pressures  than  one  with  vertical  strata  and  of  homogeneous 
structure.  Soft  sod  containing  boulders  whose  weight  develops 
local  stresses  in  the  roof  timbering  is  likely  to  be  more  danger- 
ous than  one  which  is  more  homogeneous.  A  factor  which 
greatly  increases  the  danger  of  collapse,  especially  in  soft  soils, 
is  the  presence  of  water.  This  element  often  changes  a  soil 
which  is  comparatively  stable,  when  dry,  into  one  which  is 
highly  unstable  and  treacherous.  The  liability  of  the  material 
to  disintegration  by  atmospheric  influences  and  various  other 
conditions,  which  will  occur  to  the  reader,  may  influence  its 
stability  to  a  dangerous  extent,  and  result  in  collapse. 

(2)  Collapse  is  often  the  result  of  using  defective  or  insuf- 
ficient strutting.     Of  course,  in  one  sense,  any  strutting  which 
fails  under  the  pressures    developed,  however  enormous  they 
may  be,  can  be  said  to  be  insufficient,  but  as  used  here  the  term 


268  TUNNELING 

means  a  strutting  with  an  insufficient  factor  of  safety  to  meet 
probable  increases  or  variations  in  pressure.  Insufficient  strut- 
ting may  be  due  to  the  use  of  too  light  timbers,  to  the  spacing 
of  the  roof  timbers  too  far  apart,  to  the  yielding  of  the  founda- 
tions, to  insufficient  bearing  surface  at  the  joints,  etc.  Collapse 
is  often  caused  by  the  premature  removal  of  the  strutting  dur- 
ing the  construction  of  the  masonry.  The  masons,  to  secure 
more  free  space  in  which  to  work,  are  very  likely,  unless 
watched,  to  remove  too  many  of  the  timbers  and  seriously 
weaken  the  strutting. 

(3)  The  third  cause  of  collapse  is  badly  built  masonry. 
Poor  masonry  may  be  due  to  the  use  of  defective  stone  or  brick, 
to  the  thinness  of  the  lining,  to  poor  mortar,  to  weak  centers 
which  allow  the  arch  to  become  distorted  during  construction, 
to  poor  bonding  of  the  stone  or  bricks,  to  the  premature 
removal  of  the  centers,  to  driving  some  of  the  roof  timbers 
inside  it,  etc. 

Prevention  of  Collapse,  —  Tunnels  very  seldom  collapse  with- 
out giving  some  previous  warning  of  the  possible  failure,  and 
also  of  the  manner  in  which  the  failure  is  likely  to  occur. 
From  these  indications  the  engineer  is  often  able  t.>  foresee  the 
nature  of  the  danger  and  take  steps  to  check  it.  The  danger 
may  occur  either  during  excavation  or  after  the  lining  is  built. 
During  excavation  the  danger  of  collapse  is  indicated  before- 
hand by  the  partial  crushing  or  deflection  of  the  strutting  tim- 
bers. If  the  timbers  are  too  light  or  the  bearing  surfaces  are 
too  small,  crushing  takes  place  where  the  pressures  are  the 
greatest,  and  the  timbers  bend,  burst,  or  crack  in  places,  and  the 
joints  open  in  other  places.  The  remedy  in  such  cases  is  to  in- 
sert additional  timbers  to  strengthen  the  weak  points,  or  it  may 
be  necessary  to  construct  a  double  strutting  throughout. 
When  the  distance  spanned  by  the  roof  timbers  is  too  great, 
failure  is  generally  indicated  by  the  excessive  deflection  of 
these  timbers,  and  this  may  often  be  remedied  by  inserting 
intermediate  struts  or  props.  In  some  respects  the  best  remedy 


ACCIDENTS    AND    REPAIRS    IN    TUNNELS  269 

under  any  of  these  conditions  is  to  construct  the  masonry  as 
soon  as  possible. 

When  collapse  is  likely  to  occur  after  the  masonry  is  com- 
pleted, its  probability  is  generally  indicated  by  the  cracking 
and  distortion  of  the  lining.  A  study  of  the  cause  is  quite 
likely  to  show  that  it  is  the  percolation  of  water  through  the 
material  surrounding  the  lining  which  causes  cavities  behind 
the  lining  in  some  places,  and  an  increase  of  the  pressures  in 
other  places.  When  it  is  certain  that  this  water  comes  from 
the  surface  streams  above,  these  streams  may  often  be  diverted 
or  have  their  beds  lined  with  concrete  to  prevent  further  perco- 
lation. When  percolating  water  is  not  the  cause  of  the  trouble, 
a  usually  efficient  remedy  is  to  sink  a  shaft  over  the  weak  point, 
and  refill  it  with  material  of  more  stable  character.  These, 
and  the  remedies  previously  suggested,  are  designed  to  prevent 
failure  without  resorting  to  reconstruction.  When  they  or 
similar  means  prove  insufficient,  reconstruction  or  repairs  have 
to  be  resorted  to. 

Repairing  Failures.  —  Tunnels  may  collapse  in  several  ways  : 
(1)  The  front  and  sides  of  the  excavation  may  cave  in;  (2) 
the  floor  or  bottom  may  bulge  or  sink ;  (3)  the  roof  may  fall 
in ;  (4)  the  material  above  the  entrances  may  slide  and  fill 
them  up. 

(1)  One  of  the  most  common  accidents  is  the  caving  of  the 
front  and  sides  of  the  excavation.     This  may  often  be  prevented 
by  taking  care  that  the  face  of  the  excavation -follows  the  natu- 
ral slope  of  the  material  instead  of  being  more  or  less  nearly 
vertical.     When,  however,  caving  does    occur  it  may  usually 
be  repaired  by  removing  the  fallen  material,  strongly  shoring 
the  cavity,  and  filling  in  behind  with  stone,  timber,  or  fascines. 

(2)  The  bulging  or  rising  of  the  bottom  of  the  tunnel  may 
usually  be  considered  as  a  consequence  of  the  squeezing  together 
of  the  side  walls.     It  usually  occurs  in  very  loose  soils,  and  is 
chiefly  important  from  the  fact  that  the  reconstruction  of  the 

walls  is  made  necessary.     The  sinking  of  the  tunnel  bot- 


270  TUNNELING 

torn  is  a  more  serious  occurrence.  It  seldom  happens  unless 
there  is  a  cavity  beneath  the  floor,  due  either  to  natural  causes 
or  to  the  fact  that  mining  operations  have  gone  on  in  the  hill 
or  mountain  penetrated  by  the  tunnel.  When  the  bottom  of 
the  tunnel  sinks,  three  cases  may  be  considered :  (a)  when  the 
sinking  is  limited  to  the  middle  of  the  tunnel  floor ;  (£>)  when 
only  a  portion  of  the  foundation  masonry  is  affected ;  and,  (<?) 
when  the  entire  lining  is  disturbed.  In  the  first  case  repairs 
are  easily  made  by  filling  in  the  cavity  with  new  material.  In 
the  second  case  the  unimpaired  portion  of  the  masonry  is  tem- 
porarily supported  by  shoring  while  the  injured  portion  is  re- 
moved and  rebuilt  on  a  firm  foundation.  The  remaining  cavity 
is  then  filled.  In  the  case  of  the  complete  failure  of  the  lining, 
the  method  of  repairing  employed  when  the  roof  falls,  and 
described  below,  is  usually  adopted. 

(3)  The  most  dangerous  of  all  failures  is  the  falling  of  the 
tunnel  roof.  In  such  casualties  two  cases  may  be  considered : 
(a)  When  the  falling  mass  completely  fills  the  tunnel  section, 
and  (6)  when  it  fills  only  a  portion  of  the  section. 

When  the  whole  section  is  filled  by  the  fallen  material,  the 
problem  may  be  considered  as  the  excavation  of  a  new  tunnel 
of  short  length  inside  the  old  tunnel,  and  under  rather  more 
difficult  conditions.  The  first  task,  particularly  if  men  have 
been  imprisoned  behind  the  fallen  material,  is  to  open  com- 
munication through  it  between  the  two  uninjured  portions  of 
the  tunnel.  It  is  advisable  to  do  this  even  when  there  is  no 
danger  to  life  because  of  imprisoned  workmen,  since  it  enables 
the  work  of  repairing  to  be  conducted  from  both  directions. 
The  excavation  of  a  passageway  through  the  fallen  material 
is  rendered  difficult,  both  because  the  fallen  material  is  of  an 
unstable  character,  and  also  because  it  is  usually  filled  with  the 
lining  masonry,  timbering,  etc.  When,  therefore,  the  accident 
has  happened  before  the  full  section  of  the  original  material 
has  been  removed,  the  first  heading  or  drift  is  driven  through 
this  original  material  rather  than  through  the  fallen  debris. 


ACCIDENTS    AND    REPAIRS    IN    TUNNELS 


271 


Any  of  the  regular  soft-ground  methods  of  tunneling  may  be 
employed,  but  it  is  usually  better  to  select  one  which  allows 
the  masonry  to  be  built  with  as  little  excavation  as  possible  at 
first.  For  this  reason  the  German  method  of  tunneling  is  par- 
ticularly suited  to  repair  work  of  this  nature.  The  Belgian 
method  may  also  be  used  to  advantage,  particularly  when  the 
caving  extends  to  the  surface  of  the  ground  above,  and  the 
upper  portion  of  the  debris  is,  therefore,  practically  the  same 
material  as  that  through  which  the  original  tunnel  was  driven. 
The  greatest  defect  of  the  Belgian  method  for  making  repairs 
is  that  the  roof  arch  is  supported  by  a  rather  unstable  mass  of 


FIG.  134.  —  Tunneling  through  Caved  Material  by  Heading. 

mingled  earth,  stone,  and  timber,  which  constitutes  the  bottom 
layer  of  the  fallen  material.  The  method  of  strutting  the  work 
when  the  German  or  Belgian  method  is  used  is  shown  by  Fig. 
134.  It  sometimes  happens  that  the  fallen  debris  is  so  un- 
stable that  it  will  not  carry  safely  the  arch  masonry  in  the 
Belgian  method  or  the  strutting  in  the  German  method,  and  in 
these  cases  one  of  the  full-section  methods  of  excavation  is 
usually  adopted.  The  nature  of  the  strutting  employed  is 
shown  by  Fig.  135.  When  the  section  has  been  opened  and 
the  new  masonry  built,  great  care  should  be  taken  to  fill  the 
cavity  behind  the  masonry  with  timber  or  stone ;  and  should 


272 


TUNNELING 


the  disturbance  reach  to  the  ground  surface  it  is  often  a  good 
pi  n  to  sink  a  shaft  through  the  disturbed  material,  and  fill  it 
with  more  stable  material. 


7^\  '  VT  "1=1  ^  ^ v/1 l  v  f 

FlG.  135.  — Tunneling  through  Caved  Material  by  Drifts. 

When  the  fallen  debris  fills  only  a  part  of  the  section,  the 
first  thing  to  provide  against  is  the  occurrence  of  any  further 
caving ;  and  this  is  usually  done  by  building  a  protecting  roof 
above  the  line  of  the  future  roof  masonry.  Figs.  136  and  13T 


PIGS.  136  and  137.  —Filling  in  Roof  Cavity  Formed  by  Falling  Material. 

show  two  methods  of  constructing  this  temporary  roof,  which 
it  will  be  noticed  is  filled  above  with  cordwood  packing.  As 
soon  as  the  temporary  roof  is  completed,  the  lining  masonry  is 
constructed. 


ACCIDENTS    AND    REPAIRS    IN    TUNNELS 


273 


(4)  Landslides  which  close  the  tunnel  entrance  are  repaired 
in  a  variety  of  ways.  Fig.  138  shows  a  common  method  of 
preventing  the  extension  of  a  landslide  which  has  been  started 


FIG.  138.  —Timbering  to  Prevent  Landslides  at  Portal. 

by  the  excavation  for  the  entrance  masonry.  Fig.  139  shows  a 
method  often  adopted  when  the  slope  is  quite  flat  and  the 
amount  of  sliding  material  is  small.  It  consists  essentially  of 
removing  the  fallen  material  and  building  a  new  portal  farther 
back ;  that  is,  the  open 
cut  is  extended  and  the 
tunnel  is  shortened. 
When  the  amount  of 
the  sliding  material  is 
very  large,  the  contrary  fx 

practice  of  lengthening 
the  tunnel  and  shorten- 
ing tne  open  cut,  as  7:^rf<^iprf^^ 

Shown      by      rig.     140,       FIG.  1S9.— Shortening  Tunnel  Crashed  by  Landslide 

may  be  adopted.  at  Portal. 

Accidents  After  Construction.  —  Accidents  after  the  comple- 
tion of  the  tunnel  may  be  divided  into  two  classes :  first, 
those  which  entirely  obstruct  the  passage  of  trains,  of  which  the 
collapse  of  the  roof  is  the  most  common ;  and  second,  those  which 
allow  traffic  to  be  continued  while  the  repairs  are  being  made, 


274 


TUNNELING 


such  as  the  bulging  inward  of  a  portion  of  the  lining  without 
total  collapse.  In  the  first  case  the  first  duty  of  the  engineer 
is  to  open  communication  through  the  fallen  debris,  so 
that  passengers  at  least  may  be  transferred  from  one  part  of  the 
tunnel  to  the  other  and  proceed  on  their  way.  This  is  done 
by  driving  a  heading,  and  strongly  timbering  it  to  serve  as  a 
passageway.  If  the  tunnel  is  single  tracked  this  heading  is 
afterwards  enlarged  until  the  whole  section  is  opened.  In 
double-track  tunnels  the  method  generally  adopted  is  to  open 
first  one  side  of  the  section  and  timber  it  strongly,  so  as  to  clear 
one  track  for  traffic.  While  the  trains  are  run- 
ning through  this  temporary  passageway  the 
other  half  of  the  section  is  opened  and  re- 
paired ;  the  traffic  is  then  shifted  to  the 
new  permanent  track,  and  the  temporary 
structure  first  employed  is  replaced 
with  a  permanent  lining. 
When  the  accident  is  such 
that  the  repairs  can  be 
made  without  ob- 
structing traffic  en- 


tirely,         VariOUS    FlG  ^  14Q  _  Extending  Tunnel  through  Landslide  at  Portal. 

modes  of  procedure 

are  followed.  In  all  cases  great  care  has  to  be  exercised  to 
prevent  accident  to  the  trains  and  to  the  tunnel  workmen. 
The  work  should  be  done  in  small  sections  so  as  to  disturb  as 
little  as  possible  the  already  troubled  equilibrium  of  the  soil  ; 
the  strutting  should  be  placed  so  as  to  give  ample  clearing 
space  to  passing  trains,  and  the  trains  themselves  should  be  run 
at  slow  speeds  past  the  site  of  the  repairs.  To  illustrate  the 
two  kinds  of  accidents  and  the  methods  of  repairing  them, 
which  have  been  mentioned,  the  accidents  at  the  Giovi  tunnel 
in  Italy  and  at  the  Chattanooga  tunnel  in  America  have  been 
selected. 

Giovi  Tunnel  Accident.  —  In  September,  1869,  at  a  point  about 


ACCIDENTS    AND    REPAIRS    IN    TUNNELS  '1 »  O 

220  ft.  from  the  south  portal  of  the  Giovi  tunnel,  a  disturbance 
of  the  masonry  lining  for  a  length  of  about  52  ft.  was  observed. 
Accurate  measurements  showed  that  the  lining  was  not  sym- 
metrical with  respect  to  the  vertical  axis  of  the  sectional  profile. 
It  was  concluded  that  owing  to  some  disturbance  of  the  sur- 
rounding soil  unsymmetrical  vertical  and  lateral  pressures  were 
acting  on  the  masonry.  Close  watch  was  kept  of  the  dis- 
torted masonry,  which  for  some  time  remained  unchanged 
in  position.  In  1872,  however,  new  crevices  were  observed 
to  have  developed,  and  shortly  afterwards,  in  January,  1873, 
the  injured  portion  of  the  masonry  caved  in,  obstructing 
the  whole  tunnel  section.  The  fallen  material  consisted 
chiefly  of  clay  in  a  nearly  plastic  state.  The  surface  of  the 
ground  above  was  observed  to  have  settled.  Investigation 
showed  also  that  the  cause  of  the  caving  was  the  percolation  of 
water  from  a  nearby  creek.  The  water  had  soaked  the  ground, 
and  decreased  its  stability  to  such  an  extent  that  the  masonry 
lining  was  unable  to  withstand  the  increased  vertical  and  lateral 
pressures. 

The  mode  of  procedure  decided  upon  for  repairing  the 
damage  was :  (1)  To  open  at  least  one  track  for  the  temporary 
accommodation  of  traffic  ;  (2)  To  remove  permanently  the  causes 
which  had  produced  the  collapse  ;  (3)  To  build  a  new  and 
much  stronger  lining.  Close  to  the  western  side  wall,  which 
was  still  standing,  the  debris  was  removed,  and  the  opening 
strongly  strutted  in  order  to  allow  the  laying  of  a  single 
track  to  reestablish  communication.  At  the  same  time  a  shaft 
was  sunk  from  the  surface  above  the  caved  portion  of  the  tunnel, 
for  the  double  purpose  of  facilitating  the  removal  of  the 
fallen  material  and  of  affording  ventilation.  The  depth  of  the 
surface  above  the  tunnel  was  41.6  ft.,  which  made  the  construc- 
tion of  the  shaft  a  comparatively  easy  matter.  The  shaft  itself 
was  6i  ft.  wide  and  18  ft.  long,  with  its  longer  dimensions  parallel 
to  the  tunnel,  and  it  was  lined  with  a  rectangular  horizontal 
frame  and  vertical-poling  board  construction.  After  tern- 


276  TUNNELING 

porary  communication  had  been  opened  on  the  western  track  of 
the  tunnel,  the  remainder  of  the  fallen  earth  was  removed  and 
the  excavation  strutted.  The  new  masonry  lining  was  then 
built. 

To  remove  permanently  the  cause  of  the  cave-in,  which  was 
the  percolation  of  water  from  a  close-by  stream,  this  stream  was 
diverted  to  a  new  channel  constructed  with  a  concrete  bed  and 
side  walls. 

The  failure  of  the  original  lining  occurred  by  cracks  develop- 
ing at  the  crown,  haunches,  and  springing  lines.  The  new  lining 
was  made  considerably  thicker  than  the  original  lining,  and  at 
the  points  where  failure  had  first  occurred  in  the  original  arch 
cut-stone  voussoirs  were  inserted  in  the  brickwork  of  the  new 
arch  as  described  in  Chapter  XIII. 

Chattanooga  Tunnel.  —  The  Western  &  Atlantic  Ry.  passes 
through  the  Chattanooga  mountains  by  means  of  a  single-track 
tunnel  1,477  ft.  long,  constructed  in  1848-49.  The  lining  con- 
sisted of  a  brickwork  roof  arch  and  stone  masonry  side  walls. 
After  the  tunnel  had  been  opened  to  traffic,  this  lining  bulged 
inward  at  places,  contracting  the  tunnel  section  to  such  an  ex- 
tent that  it  was  decided  to  reconstruct  the  distorted  portions. 
After  careful  surveys  and  calculations  had  been  made,  it  was 
decided  to  take  down  and  reconstruct  about  170  ft.  of  the 
lining. 

Owing  to  contracted  space  in  the  tunnel,  it  was  necessary 
to  remove  all  men,  tools,  and  material,  whenever  trains  were 
to  pass  through ;  and  in  order  to  do  this  a  work-train  of 
three  cars  was  fitted  up  with  necessary  scaffolds,  arid  supplied 
with  gasoline  torches  for  lighting  purposes.  Mortar  was  mixed 
on  the  cars,  and  all  material  remained  on  them  until  used. 
Debris  torn  out  of  the  old  wall  was  loaded  on  the  cars,  and 
hauled  to  the  waste  dump.  A  siding  was  built  near  the  West 
end  of  the  tunnel  for  the  use  of  this  train,  and  a  telephone  sys- 
tem was  installed  between  the  entrances  and  the  working-train. 
On  account  of  the  contracted  working-space  and  the  greater 


ACCIDENTS    AND    KEPAIKS    IN    TUNNELS  277 

ease  with  which  brick  could  be  handled,  it  was  decided  to  re- 
build the  walls  out  of  brick  instead  of  stone. 

In  tearing  out  the  old  wall  a  hole  was  first  cut  through  the 
three  bottom  courses  of  the  arch  and  gradually  widened.  When 
the  opening  became  four  or  five  feet  long,  a  small  jack  was 
placed  near  the  center  of  it  and  brought  to  a  bearing  against 
the  arch  to  sustain  it.  After  cutting  the  opening  to  a  length 
of  from  7  to  10  ft.  depending  on  the  stability  of  the  earth 
backing,  the  jack  was  removed  and  a  piece  of  8x16  in.  timber 
placed  under  the  arch  and  brought  up  to  a  bearing  with  jacks. 
One  end  of  the  timber  rested  on  the  old  wall,  the  other  on  a  seat 
built  into  the  adjoining  section  of  new  wall.  Wedges  were 
then  driven  under  the  ends  of  timber  and  the  jacks  removed. 
With  this  timber  in  place,  the  old  wall  could  be  taken  down 
with  ease,  the  only  trouble  being  that  small  stones  and  earth 
fell  in  from  above  and  behind  the  arch.  This  was  obviated 
by  placing  a  2  in.  plank  across  the  opening  and  just  back  of 
the  8x16  in.  timber.  At  several  points,  however,  the  earth 
backing  was  saturated  with  water,  and  it  became  necessary  to 
put  in  lagging  as  the  old  wall  was  removed.  This  timbering 
would  be  taken  out  as  the  new  work  was  built  up. 

A  suitable  foundation  for  the  new  wall  was  secured  at  a 
depth  from  2  to  4  ft.,  and  a  concrete  footing  was  used.  The 
section  of  the  new  wall  was  then  built  up  as  near  as  possible  to 
the  8x16  in.  timber ;  the  timber  was  then  removed  and  the 
new  wall  built  up  and  keyed  under  the  arch. 

The  new  wall  had  a  minimum  width  of  2j  ft.  at  the  top, 
and  4  ft.  at  the  base  of  rail,  and  was  provided  with  weep  holes 
at  intervals.  To  facilitate  matters,  work  was  carried  on  simul- 
taneously at  two  or  three  different  places,  the  intention  being 
to  get  one  place  torn  out  and  ready  for  the  bricklayers  by  the 
time  they  completed  a  section  of  the  new  wall  at  another 
place. 

In  rebuilding  the  arch,  sections  extending  from  the  spring- 
ing line  up  as  far  as  was  necessary  to  obtain  the  desired  clear- 


278  TUNNELING 

ance,  and  from  2J  to  4  ft.  in  length,  were  removed.  Near  the 
sides,  the  Dearth  above  the  arch  was  a  stiff  clay,  which  was  self- 
sustaining;  but  near  the  center  there  occurred  a  stratum  of 
gravel  and  clay  saturated  with  water.  This  gave  considerable 
trouble,  falling  through  almost  continuously  until  timbering 
could  be  placed.  One  end  of  this  timber  rested  on  the  old 
arch,  the  other  on  the  adjoining  section  of  the  new  work.  As 
the  new  work  was  to  be  set  6  to  13  ins.  back  from  the  old,  it 
was  necessary  to  block  up  this  distance  on  top  of  the  old  arch, 
to  carry  the  end  of  the  lagging  timber,  in  order  that  the  timber 
should  be  clear  of  the  new  arch. 

Owing  to  the  small  clearance  between  the  car  roof  and  the 
arch,  a  special  form  of  centering  was  required,  one  that  would 
occupy  as  small  space  as  possible.  Bar  iron  1  in.  thick,  4  ins. 
wide,  and  20  ft.  long  was  curved  to  a  radius  of  6£  ft.,  and  on 
the  underside  of  this  was  riveted  a  6-in.  plate  1  in  thick.  This 
plate  projected  1  in.  on  the  sides  of  the  centering,  and  carried 
the  ends  of  the  1  in.  boards  used  for  lagging.  The  rivets  were 
counter-sunk  on  the  outside  of  the  centering  to  present  a  smooth 
surface  next  the  arch. 

In  keying  up  a  section  of  the  new  work,  a  space  about  18  ins. 
square  had  to  be  left  open  for  the  use  of  the  workmen.  As 
soon  as  the  next  section  had  been  torn  out,  this  space  was  built 
up.  In  building  up  the  last  section,  this  space  had  to  be  filled 
from  below,  which  proved  to  be  a  tedious  undertaking.  The 
opening  was  gradually  reduced  to  a  size  of  10  x  18  in.,  and  the 
top  ring  then  completed  and  keyed  up,  the  adhesion  of  mortar 
holding  the  bricks  in  place  until  the  key  could  be  driven  home. 
The  next  ring  was  treated  in  a  similar  manner,  and  so  on  to  the 
face  ring.  Altogether  412  lin.  ft.  of  the  walls  and  178  lin.  ft. 
of  the  arch  were  taken  down  and  rebuilt,  amounting  in  all  to 
607  cu.  yds.  of  masonry  at  the  total  cost  of  $7,440,  or  about 
$12.25  per  cu.  yds. 

The  regular  trains  arrived  so  frequently  at  the  tunnel  that 
slightly  over  two  hours  was  the  longest  working-time  between 


ACCIDENTS    AND    REPAIRS    IN    TUNNELS  279 

any  two  trains,  and  usually  less  than  one  hour  at  a  time  was  all 
that  it  could  be  worked.  In  addition  to  the  regular  trains,  a 
large  number  of  extra  trains,  moving  troops,  had  to  be  accom- 
modated. Work  was  in  progress  eight  months,  and  during  that 
time  there  was  no  delay  to  a  passenger  train.  The  repairs  were 
completed  in  August,  1899.  The  work  was  under  the  direction 
of  Mr.  W.  H.  Whorley,  engineer  of  the  Western  &  Atlantic 
R.  R.,  and  foreman  of  construction,  A.  H.  Richards.  A  recent 
examination  failed  to  reveal  any  sign  of  settlement  cracks  at  the 
junction  points  of  the  new  and  old  work. 


280  TUNNELIN.G 


CHAPTER   XXIII. 

RELINING  TIMBER   LINED   TUNNELS   WITH 
MASONRY. 


THE  original  construction  of  many  American  railway  tunnels 
with  a  timber  lining  to  reduce  the  cost  and  hasten  the  work  has 
made  it  necessary  to  reline  them,  as  time  has  passed,  with  some 
more  permanent  material.  In  most  cases  the  work  of  removing 
the  old  lining  and  replacing  it  with  the  new  masonry  has  had 
to  be  done  without  interfering  with  the  running  of  trains,  and  a 
number  of  ingenious  methods  have  been  developed  by  engineers 
for  accomplishing  this  task.  Three  of  these  methods  which 
have  been  employed,  respectively,  in  relining  the  Boulder 
tunnel  on  the  Montana  Central  Ry.,  in  Montana,  the  Mullan 
tunnel  on  the  Northern  Pacific  Ry.,  in  Montana,  and  the  Little 
Tom  tunnel  on  the  Norfolk  &  Western  R.  R.,  in  Virginia,  have 
been  selected  as  fairly  representative  of  this  class  of  tunnel 
work. 

Boulder  Tunnel.  —  This  tunnel  penetrates  a  spur  of  the  main 
range  of  the  Rocky  Mountains,  at  an  elevation  at  the  summit 
of  grade  of  5,454  ft.,  and  is  6,112  ft.  in  length.  Its  alinement  is 
a  tangent,  with  the  exception  of  150  ft.  of  30'  curve  at  the 
north  end.  The  material  penetrated  is  blue  trap-rock  with 
seams  for  4,950  ft.  from  the  north  end,  and  syenitic  boulders 
with  the  intervening  spaces  filled  with  disintegrated  material 
for  the  remaining  1,160  ft.  The  dimensions  and  character  of 
the  old  timber  lining  and  of  the  new  masonry  lining  replacing 
it  are  shown  in  Figs.  141  and  142. 

The  form  of  masonry  adopted  consisted  of  coarse  rubble  side 
walls  of  granite,  13  ft.  8  ins.  high,  and  generally  20  ins.  thick, 


RELINING    TIMBER-LINED    TUNNELS    WITH   MASONRY         281 

with  a  full  center  circular  arch  of  four  rings  of  brick  laid  in 
rowlock  form.  When  greater  strength  was  needed  the  thick- 
ness of  the  side  walls  was  increased  to  30  ins.  and  that  of  the 
arch  to  six  rings  of  brick. 

The  first  plan  adopted  in  putting  in  the  masonry  was  to 
remove  all  the  timbering ;  but  owing  to  the  large  number  of 
falls  and  slides  this  was  abandoned,  and  the  plan  followed  was  to 
leave  in  the  three  roof  segments  of  the  timbering  with  the  over- 
lying cord-wood  packing  and  debris.  In  carrying  on  the  work 
the  first  step  was  to  remove  the  side  timbers.  This  was  done 
by  supporting  the  roof  timbers,  as  shown  in  Fig.  141 ;  that  is, 
the  first  and  fourth  arch  rib  of  an  8-ft.  section  containing  four 


Cross  Section. 


Longitudinal  Section. 

Cross  Section. 

FIGS.  141  and  142.  —  Relining  Timber-Lined  Tunnel. 


Cross  Section. 


arch  ribs  were  supported  by  temporary  posts.  The  intermedi- 
ate arch  ribs  were  supported  against  the  downward  pressure  by 
6  X  6  in.  timbers,  extending  from  the  side  ribs  near  the  tops 
of  the  temporary  posts  to  the  opposite  sides  of  the  intermediate 
roof  segments,  as  shown  in  the  longitudinal  section,  Fig.  142. 
To  resist  the  pressure  from  the  sides,  4  x  6  in.  braces  were 
placed  across  the  tunnel  from  near  the  center  of  the  intermedi- 
ate segments  to  the  upper  ends  of  the  hip  segments,  as  shown 
in  the  cross-section,  Fig.  141.  The  hip  segments  were  then 
sawed  off  below  the  notch,  and  the  side  timbering  removed  and 
the  masonry  built. 

The  stone  was  conveyed  into  the  tunnel  on  flat  cars,  and  laid 
by  means  of  small  derricks  located  on  the  cars.     Two  derricks 


282 


TUNNELING 


were  used,  one  for  each  side  wall,  and  the  work  on  both  walls 
was  carried  011  simultaneously. 

The  arch  was  built  upon  a  centering,  the  ribs  of  which  were 
54  ins.  less  in  diameter  than  the  distance  between  the  side 
walls,  so  as  to  permit  the  use  of  2|  ins.  lagging.  Each  center 
had  three  ribs,  made  in  1-in.  or  2-in.  board  segments,  10  ins.  thick 
and  14  ins.  deep.  These  ribs  were  mounted  on  frames,  which 
followed  the  opposite  walls,  and  were  4  ft.  apart,  making  the 
total  length  of  the  center  out  to  out  about  9  ft.  The  frames, 
upon  which  the  ribs  were  supported,  are  shown  in  Fig.  143. 
As  will  be  seen,  they  were  mounted  on  dolly s  to  enable  the 
center  to  be  moved  from  one  section  to  another.  Jacks  were 

used  to  raise  and  lower 
the  center  into  its  proper 
position. 

The  arch  was  built  up 
from  the  springing  lines 
on  both  sides  at  the  same 
time,  four  masons  being 
employed.  The  rings 
were  built  beginning  with 
the  intrados,  which  was 


Cross  Section. 


Longitudinal  Section. 

FIG.  143.  — Relining  Timber-Lined  Tunnel, 
Great  Northern  Ry. 

brought  up,  say,  a  dis- 
tance of  about  2  ft.  from  the  springing  line.  Then  the  back  of 
the  ring  was  well  plastered  with  from  j  in.  to  4  in.  of  mortar, 
and  the  second  ring  brought  up  to  the  same  height  and 
plastered  on  the  back,  and  so  on  until  the  last  ring  was  laid. 
After  bringing  the  full  width  of  the  arch  up  some  distance, 
new  laggings  were  placed  on  the  ribs  for  an  additional  height 
of  2  ft.  and  the  same  process  was  repeated.  All  the  space 
between  the  extrados  of  the  masonry  arch  and  the  old  lining 
was  compactly  filled  with  dry  rubble.  When  high  enough 
so  that  the  hip  segments  had  a  foot  or  more  bearing  on  the 
masonry  the  segments  were  securely  wedged  and  blocked  up 
against  the  brickwork,  and  the  longitudinal  4  X  6  in.  timbers 


RELINING    TIMBER-LINED    TUNNELS    WITH    MASONRY       283 


removed.  The  remaining  space  was  now  clear  for  completion 
of  the  arch,  and  both  sides  were  brought  up  until  there  wa& 
not  sufficient  space  for  four  masons  to  work,  when  the  keying 
was  completed  by  two  masons  beginning  at  the  completed  and 
working  back  toward  the  toothed  end.  The  brickwork  was 
built  from  the  top  of  a  staging-car. 

In  a  few  instances  where  slides  occurred  after  the  removal 
of  the  slide  timbering,  the  method  of  re  timbering  the  tunnel 
shown  in  Fig.  144  was  adopted.  Two  side  drifts  were  first 
run  2|  ft.  wide  by  4  ft.  high,  and  the  plate  timbers  placed  in 
position  and  blocked.  Cross  drifts  were  then  run,  and  the  roof 
segments  placed,  and  the  core  down  to  the  level  of  the  bottoms 
of  the  side  drifts  taken 
out.  The  lower  wall 
plates  were  then  placed 
and  the  hip  segments  Jj 
inserted.  The  bench 
was  then  taken  down 
by  degrees,  the  side 
plates  being  held  by 
jacks,  and  the  posts 
placed  one  at  a  time. 
As  the  masonry  at  the 
points  where  slides  occur  consists  of  30-in.  walls  and  six-ring 
arch,  the  timbering  was  22  ft.  wide  in  the  clear,  with  other 
dimensions  as  shown  in  Fig.  144. 

Only  a  single  crew  of  brick  and  stone  masons  was  employed. 
In  order  to  prepare  the  sections  for  these  masons  it  was 
necessary  to  have  timber  and  trimming  crews  at  work  through- 
out the  whole  day  of  24  hours,  so  that  an  engine  and  two  train 
crews  were  in  constant  attendance.  The  single  mason  crews 
were  able  to  complete  8  ft.  of  side  wall  and  arch  in  24  hours. 
The  number  of  men  actually  employed  at  the  tunnel  was  35. 
This  included  electric-light  maintenance,  and  all  other  labor 
pertaining  to  the  work.  The  tunnel  was  lighted  by  an  Edison 


•  7.83  •  •••*;•••  7.83"  •  •> 

Cross  Section.  Longitudinal  Section. 

FIG.  144.  —  Kelining  Timber-Lined  Tunnel, 
Great  Northern  Ky. 


284 


TUNNELING 


and  has  a  grade  of  20 


dynamo  of  20  arc  light  capacity,  one  arc  light  being  placed  on 
each  side  of  the  tunnel  at  all  working-places.  Each  lamp 
carried  a  coil  of  wire  20  or  30  ft.  long  to  allow  it  to  be  shifted 
from  place  to  place  without  delay. 

Mullan  Tunnel.  —  This  tunnel  is  3,850  ft.  long,  and  crosses 
the  main  range  of  the  Rocky  Mountains,  about  20  miles 
west  of  Helena,  Mont.  The  tunnel  is  on  a  tangent  throughout, 

falling  toward  the  east.  The  summit 
of  the  grade,  west  of  the  tun- 
nel, is  5,548  ft.  above  sea 
level,  and  the  mountain  above 
the  line  of  the  tunnel  rises 
to  an  elevation  of  5,855  ft. 
Owing  to  the  treacherous 
nature  of  the  material  through 
which  the  tunnel  passed,  it 
had  been  a  constant  menace 
to  traffic  ever  since  its  con- 
struction in  1883,  and  numer- 
ous delays  to  trains  had  been 
caused  by  the  falls  of  rock 
and  fires  in  the  timber  lin- 


Permanent  Work. 


FIG.  145. 


Relining  Timber  Lined  Tunnel, 
Great  Northern  Ry. 


ing.    For  these  reasons  it  was 
finally  decided  to  build  a  per- 
manent masonry  lining,  and 
work  on  this  was  begun  in  July,  1892. 

The  original  timbering  consisted  of  sets  spaced  4  ft.  apart 
c.  to  <?.,  with  12  x  12  in.  posts  supporting  wall  plates,  and  a 
five-segment  arch  of  12  x  12  in.  timbers  joined  by  H-in. 
dowels.  The  arch  was  covered  with  4-in.  lagging,  and  the 
space  between  this  and  the  roof  was  filled  with  cordwood. 
Except  where  the  width  had  been  reduced  by  timbering  placed 
inside  the  original  timbering  to  increase  the  strength,  the  clear 
width  was  16  ft.,  and  the  clear  height  20  ft.  above  the  top  of 
the  rail.  Fig.  145  shows  the  timbering  and  also  the  form 


RELINING    TIMBER-LINED   TUNNELS    WITH   MASONRY       285 


of  masonry  lining  adopted.  The  side  walls  are  of  concrete  and 
the  arch  of  brick.  This  new  masonry,  of  course,  required  the 
removal  of  all  the  original  timbering.  The  manner  of  doing 
this  work  is  as  follows  :  A  7-ft  section,  A  B,  Fig,  146,  was  first 
prepared  by  removing  one  post  and  supporting  the  arch  by 
struts,  £  &  After  clearing  away  any  backing,  and  excavating  for 
the  foundation  of  the  side  wall,  two  temporary  posts,  F  F,  were 
set  up,  and  fastened  by  hook  bolts,  Fig.  146,  Z,  and  a  lagging 
was  built  to  form  a  mold  for  the  concrete.  Several  of  these 
7-ft.  sections  were  prepared  at  a  time,  each  two  being  sepa- 
rated by  a  5-ft.  section  of  timbering. 


M 


With  Wall  Pate .  Without  Wol]  Plate. 

SecKon  .with  Concrete  Car.  Longitudinal  Sectjon. 

FIG.  146.  —  Construction  of  Centering  Mullan  Tunnel. 

The  mortar  car  was  then  run  along,  and  enough  mortar 
(1  cement  to  3  sand)  was  run  by  the  chute  into  each  section 
to  make  an  8-in.  layer  of  concrete.  As  the  car  passed  along 
to  each  section,  broken  stone  was  shoveled  into  the  last  preced- 
ing section  until  all  the  mortar  was  taken  up.  The  walls  were 
thus  built  up  in  8-in.  layers,  and  became  hard  enough  to  sup- 
port the  arches  in  about  10  to  14  days.  The  arches  were  then 
allowed  to  rest  on  the  wall,  and  the  posts  of  the  remaining  5-ft. 
sections  were  removed,  and  the  concrete  wall  built  up  in  the 
same  way  as  before. 


286 


TUNNELING 


The  average  progress  per  working-day  was  30  ft.  of  side 
wall,  or  about  45  cu.  yds. ;  and  the  average  cost,  including  all 
work  required  in  removing  the  timber  work,  train  service,  lights 
and  tools,  engineering  and  superintendence,  and  interest  on 
plant,  was  $8  per  cubic  yard. 

The  centering  used  for  putting  in  the  brick  arches  is  shown 
in  Fig.  147.  From  3  ft.  to  9  ft.  of  arch  was  put  in  at  a  time, 
the  length  depending  upon  the  nature  of  the  ground.  To  re- 
move the  old  timber  arch,  one  of  the  segments  was  partly  sawed 
through ;  and  then  a  small  charge  of  giant  powder  was  exploded 

in  it,  the  resulting  debris, 
cordwood,  rock,  etc.,  being 
caught  by  a  platform  car  ex- 
tending underneath.  From 
this  car  the  debris  was  re- 
moved to  another  car,  which 
conveyed  it  out  of  the  tunnel. 
The  center  was  then  placed 
and  the  brickwork  begun,  the 
cement  car  shown  in  Fig.  146 
being  used  for  mixing  the 
mortar.  The  size  of  the 
bricks  used  was  2^  +  2^  +  9 
ins.,  four  rings  making  a  20- 

in.  arch  and  giving  1.62  cu.  yds.  of  masonry  in  the  arch  per 
lin.  ft.  of  tunnel.  The  bricks  were  laid  in  rowlock  bond,  two 
gangs,  of  three  bricklayers  and  six  helpers  each,  laying  about  12 
lin.  ft.  per  day.  The  brickwork  cost  about  $17  per  cu.  yd. 
The  total  cost  of  the  new  lining  averaged  about  $50  per  lin.  ft. 
Little  Tom  Tunnel.  —  The  tunnel  has  a  total  length  of  1,902 
ft.,  but  only  1,410  ft.  of  it  were  originally  lined  with  timber. 
This  old  timber  lining  consists  of  bents  spaced  3  ft.  apart,  and 
located  as  shown  by  the  dotted  lines  in  the  cross-section,  Fig. 
148.  Instead  of  renewing  this  timber,  it  was  decided  to  replace 
it  with  a  brick  lining.  Although  the  tunnel  was  constructed 


FIG.  147.  —  Centering  Mullan  Tunnel. 


REUNING    TIMBER-LINED    TUNNELS    WITH    MASONRY      287 


•288 


TUNNELING 


through  rock,  this  rock  is  of  a  seamy  character,  and  in  some 
portions  of  the  tunnel  it  disintegrates  on  exposure  to  the  air. 
In  removing  the  timber  to  make  place  for  the  new  lining  some 
of  the  roof  was  found  close  to  the  lagging,  but  often  also  con- 
siderable sections  showed  breakages  in  the  roof  extending  to  a 
height  varying  from  1  ft.  to  12  ft.  above  the  upper  side  of  the 
timbering.  This  dangerous  condition  of  the  roof  made  it  neces- 


FlG.  149.  —  Kelining  Timber-Lined  Tunnel,  Norfolk  and  Western  Ry. 

sary  that  only  a  small  section  of  the  timber  lining  should  be 
removed  at  one  time.  It  made  it  necessary,  also,  that  the  brick 
arch  should  be  built  quickly  to  close  this  opening,  and  finally 
that  all  details  of  centers,  etc.,  should  be  arranged  so  as  to 
furnish  ample  clearance  to  trains.  The  accompanying  illustra- 
tions show  the  solution  of  the  problem  which  was  arrived  at. 
Referring  to  the  transverse  and  longitudinal  sections  shown 


RELLNING    TIMBER-LINED    TUNNELS    WITH    MASONRY       289 

by  Fig.  148,  it  will  be  seen  that  two  side  trestles  were  built  to 
carry  an  adjustable  centering  for  the  roof  arch.  Two  sections 
of  these  trestles  and  centerings  were  used  alternately,  one  being 
carried  ahead  and  set  up  to  remove  the  timbering  while  the 
masons  were  at  work  on  the  other.  The  manner  of  setting  up 
and  adjusting  the  trestles  and  centerings  is  shown  by  Fig.  148 
and  also  by  Fig.  149,  which  is  an  enlarged  detail  drawing  of 
the  set  screw  and  rollers  for  the  centering  ribs.  The  following 
is  the  bill  of  material  required  for  one  set  of  trestles  and  one 
center : 

Trestles : 

Caps  and  sills 8  pieces  8  x  8  ins.  x  20  ft. 

Posts 18      "      8  x  8    "    x  11  " 

Braces 16      "      6  x  4    "    x    7  " 

Centerings : 

Ribs ,     ....  27  "  2  x  18  "  x    7  " 

Bracing 12  "  2  x    8  "  x    7  " 

Support  to  crown  lagging    .     ....  2  "  6  x    6  "  x  10  " 

Crown  lagging .     .     .  20  "  3  x    6  "  x    2  " 

Side  lagging 30  "  3  x    6  "  x  10  " 

Side  strips 2  "  2  x  12  "  x    9  " 

Blocking  for  rollers   .......  1  "  5  x    8  «  x  12  " 

6  screw  and  roller  castings  complete  with  bolts  and  lever;  114 
bolts  l-ins.  in  diameter ;  7i  U.  H.  hexagonalnut  and  2  cast  washers 
each. 

With  this  arrangement  the  progress  made  per  day  varied 
from  2  1  in.  ft.  to  3  lin.  ft.  of  lining  complete.  By  work  com- 
plete is  meant  the  entire  lining,  including  stone  packing  between 
the  brickwork  and  the  rock.  On  Feb.  23,  1900,  363  ft.  of  lin- 
ing had  been  completed,  at  a  cost  of  $ 33.50  per  lin.  ft.  This 
cost  includes  the  cost  of  removing  the  old  timber,  the  loose  rock 
above  it,  and  all  other  work  whatsoever. 


290  TUNNELING 


CHAPTER  XXIV. 

THE     VENTILATION    AND    LIGHTING     OF    TUN- 
NELS  DURING  CONSTRUCTION. 


VENTILATION. 

IN  long  tunnels,  especially  when  excavated  in  hard  rock, 
proper  ventilation  is  of  great  importance,  because  the  air  cannot 
be  easily  renewed,  and  the  amount  of  oxygen  consumed  by 
miners'  horses  and  lamps  during  construction  is  very  large. 
The  gases  produced  by  blasting  also  tend  to  fill  the  head  of  ex- 
cavation with  foul  air.  Pure  atmospheric  air  contains  about 
21  %  of  oxygen  and  only  0.04  %  of  carbonic  acid ;  when  the 
latter  gas  reaches  0.1  %,  the  fact  is  indicated  by  the  bad  odor; 
at  0.3  %  the  air  is  considered  foul,  and  when  it  reaches  0.5  %  it 
is  dangerous.  It  is  generally  admitted  that  the  standard  of 
purity  of  the  air  is  when  it  contains  0.08  °fc  of  carbonic  acid. 

A  large  quantity  of  carbonic  acid  in  the  air  is  easily  detected 
by  observing  the  lamps,  which  then  give  out  a  dim  red  light 
and  smoke  perceptibly ;  the  workmen  also  suffer  from  headache 
and  pains  in  the  eyes,  and  breathe  with  difficult}^.  Naturally, 
miners  cannot  easily  work  in  foul  air  and,  therefore,  make  very 
slow  progress.  It  is,  therefore,  to  the  interest  of  the  engineer  to 
afford  good  ventilation,  not  only  because  of  his  duty  to  care  for 
the  safety  and  health  of  his  men,  but  also  for  reasons  of  econ- 
omy, so  that  the  men  may  work  with  the  greatest  possible  ease, 
thus  assuring  the  rapid  progress  of  the  work. 

It  would  be  impossible  to  change  completely  the  atmosphere 
inside  a  tunnel,  as  the  gases  developed  from  blasting  will  pene- 
trate into  all  the  cavities  and  gather  there,  but  the  fresh  air 


VENTILATION    AND    LIGHTING   DURING    CONSTRUCTION       291 

carried  inside  by  ventilation  has  a  very  small  percentage  of  car- 
bonic acid,  mixes  with  that  which  contains  a  greater  quantity, 
and  dilutes  it  until  the  air  reaches  the  standard  of  purity.  We 
have  not  here  considered  the  gases  developed  from  the  decom- 
position of  carboniferous  and  sulphuric  rocks,  which  may  be 
met  with  in  some  tunnels,  and  which  render  ventilation  still 
more  necessary.  Tunnels  may  be  ventilated  either  by  natu- 
ral or  artificial  means. 

Natural  Ventilation.  —  It  is  well  known  that  if  two  rooms  of 
different  temperatures  are  put  in  communication  with  each 
other,  e.g.,  by  opening  a  door,  a  draft  from  the  colder  room  will 
enter  the  other  from  the  bottom,  and  a  similar  draft  at  the  top, 
but  with  a  contrary  direction,  will  carry  the  hot  air  into  the 
colder  room,  thus  producing  perfect  ventilation,  until  the  two 
rooms  have  the  same  temperature.  Now,  during  the  construc- 
tion of  tunnels  the  temperature  inside  may  be  considered  as 
constant,  or  independent  of  the  outside  atmospheric  variations ; 
hence  during  summer  and  winter,  there  will  always  be  a  draft 
affording  ventilation,  owing  to  the  difference  of  temperature  in- 
side and  outside  the  tunnel.  In  winter  time  the  cold  air  out- 
side will  enter  at  the  bottom  of  the  entrances  and  headings,  or 
along  the  sides  of  the  shafts,  and  the  hot  air  will  pass  out  near 
the  top  of  the  headings  or  entrances  or  the  center  of  the  shafts ; 
in  summer  the  air  currents  will  take  the  contrary  direction. 

Natural  ventilation  in  tunnels  is  improved  when  the  exca- 
vation of  the  heading  reaches  a  shaft,  because  the  interior  air 
can  then  communicate  with  the  exterior  at  two  points,  at  dif- 
ferent levels.  In  such  cases  a  force  equal  to  the  difference  in 
weight  between  a  column  of  air  in  the  shaft  and  a  similar  one 
of  different  density  at  the  entrance  of  the  tunnel,  will  act  upon 
the  mass  of  air  in  the  tunnel  and  keep  it  in  movement,  thus 
producing  ventilation.  Consequently,  during  whiter,  when  the 
outside  air  has  greater  weight  than  that  inside,  the  air  will 
come  in  by  the  headings  and  go  out  by  the  shaft,  and  in  the 
summer  it  will  enter  at  the  shaft  and  pass  out  at  the  entrance. 


292  TUNNELING 

Sometimes  to  afford  better  ventilation  shafts  8  or  12  in.  in  di- 
ameter are  sunk  exclusively  for  the  purpose  of  changing  the 
air.  When  the  inside  temperature  is  equal  to  that  outside, 
as  often  happens  during  the  spring  and  autumn,  there  are  no 
drafts,  and  consequently  the  air  in  the  excavation  is  not  re- 
newed and  becomes  foul ;  then  fires  are  lighted  under  the 
shaft  and  a  draft  is  artificially  produced.  The  hot  air  going 
out  through  the  shaft,  as  through  a  chimney,  allows  the  fresh 
air  to  come  in  as  in  ordinary  ventilation. 

When  the  head  of  the  excavation  is  very  far  from  the  en- 
trances, or  when  the  mountain  is  too  high  to  allow  excavation 
by  shafts,  it  is  quite  impossible  to  secure  good  natural  ventila- 
tion, especially  during  the  spring  and  autumn  months,  and  the 
engineer  has  to  resort  to  some  artificial  means  by  which  to 
supply  fresh  air  to  the  workmen. 

Artificial  Ventilation.  —  Artificial  ventilation  in  tunnels  may 
be  obtained  in  two  different  ways,  known  as  the  vacuum  and 
plenum  methods.  Their  characteristic  difference  consists  in 
this,  that  in  the  vacuum  method  the  air  is  drawn  from  the  in- 
side and  the  vacuum  thus  produced  causes  the  fresh  air  from 
the  outside  to  rush  in,  while  the  plenum  method  consists  in 
forcing  in  the  fresh  air  which  dilutes  the  carbonic  air  produced 
inside  the  tunnel  by  workingmen  and  explosives.  In  the  vac- 
uum method  the  pressure  of  the  atmosphere  inside  the  tunnel  is 
always  less  than  the  pressure  outside,  while  in  the  plenum 
method  the  pressure  within  is  always  greater  than  that  outside. 
Ventilation  is  the  result  of  this  difference  of  pressure,  as  the 
tendency  of  the  air  toward  equilibrium  produces  continuous 
drafts.  Both  these  methods  have  their  advantages  and  disad- 
vantages ;  but  in  the  presence  of  hard  rock,  when  explosives  are 
continually  required,  the  vacuum  method  is  considered  the  best, 
because  the  gases  attracted  to  the  exhaust  pipes  are  expelled 
without  passing  through  the  whole  length  of  the  tunnel,  thus 
avoiding  the  trouble  that  a  draft  of  foul  air  will  give  to  the 
workmen  who  are  within  the  tunnel.  In  both  these  methods  it 


' 


VENTILATION    AND    LIGHTING    DURING    CONSTRUCTION       293 

is  necessary  to  separate  the  fresh  air  from  the  foul  one  ;  and  this 
is  done  by  means  of  pipes  which  will  exhaust  and  expel  the 
foul  air  in  the  vacuum  method,  or  force  to  the  front  a  current 
of  fresh  air  when  the  plenum  method  is  used.  Artificial  venti- 
lation may  also  be  obtained  by  compressed  air  which  is  set  free 
after  it  has  driven  the  machines,  especially  in  tunnels  excavated 
through  rock,  when  rock  drilling  machines  moved  by  com- 
pressed air  are  employed. 

Vacuum  Method  Contrivances.  —  The  most  common  of  the  vac- 
uum appliances  consists  in  the  simple  arrangement  of  a  pipe 
leading  from  the  head  of  the  tunnel  out  through  the  fire  of  a 
furnace.  The  air  in  the  pipe  is  rarefied  by  the  heat  of  the  fur- 
nace and  then  set  free  from  the  other  end  of  the  pipe,  thus 
creating  a  partial  vacuum  in  the  pipe,  into  which  the  foul  air  of 
the  head  rushes,  the  fresh  air  from  the  entrance  taking  its  place, 
and  thus  ventilating  the  tunnel.  A  similar  arrangement  may 
be  used  with  shafts,  and  the  foul  air  may  be  driven  out  by  a 
furnace  which  is  placed  either  at  the  top  or  bottom  of  the  shaft. 
Such  furnaces  act  the  same  as  those  commonly  used  for  heating 
purposes  in  the  houses,  with  this  difference,  that,  instead  of  fresh 
air  being  forced  in,  foul  air  is  expelled.  Another  simple 
arrangement  for  producing  a  vacuum  is  by  means  of  a  steam 
jet  which  is  thrown  into  the  pipe,  and  which  helps  the  expul- 
sion of  the  air  by  heating  it,  thus  producing  a  different  density 
which  originates  a  draft  besides  that  mechanically  originated  by 
the  force  of  the  steam  jet,  which  tends  to  carry  out  the  foul  air 
of  the  pipes. 

Foul  air  may  also  be  expelled  by  means  of  exhaust  fans 
which  are  connected  with  pipes  near  the  entrance  of  the  tunnel. 
The  fan  consists  of  a  box  containing  a  kind  of  a  paddle  wheel 
turned  by  steam  or  water  power  and  arranged  so  as  to  revolve 
at  a  high  speed.  The  air  inside  the  pipe  is  forced  out  by 
blades  attached  to  the  wheel,  and  thus  the  foul  air  of  the  front 
is  driven  away  and  fresh  air  from  the  entrance  rushes  in  to  take 
its  place,  and  perfect  ventilation  is  obtained. 


294  TUNNELING 

The  best  manner  of  expelling  foul  air  from  tunnels,  accord- 
ing to  the  vacuum  method,  is  by  means  of  bell  exhausters. 
This  consists  of  two  sets  of  bells  connected  by  an  oscillating 
beam  and  balancing  each  other.  Each  set  consists  of  a  movable 
bell,  which  covers  and  surrounds  a  fixed  bell  with  a  water  joint. 
In  the  central  part  of  the  fixed  bell  there  are  valves  which  open 
upwards,  and  on  the  bottom  of  each  movable  bell  there  are 
valves  which  open  from  the  outside.  When  one  bell  ascends, 
the  valves  at  the  bottom  are  closed,  the  air  beneath  is  then 
rarefied,  and  a  vacuum  is  produced ;  the  valves  in  the  central 
part  of  the  fixed  bell  filled  with  water  are  opened,  and  there  is 
an  aspiratory  action  from  the  pipe  leading  to  the  headings,  and 
the  foul  air  is  thus  carried  away.  The  apparatus  makes  about 
ten  oscillations  per  minute,  and  the  dimensions  of  the  bells 
depend  upon  the  quantity  of  air  to  be  exhausted  in  a  minute. 
In  the  St.  Gothard  tunnel,  where  these  bell  exhausters  were 
used,  they  exhausted  16,500  cu.  ft.  of  air  per  minute. 

Plenum  Method  Contrivances Fresh  air  may  be  driven  into 

tunnels  to  dilute  the  carbonic  acid  by  two  different  ways,  viz., 
by  water  blast  and  by  fans.  Water  when  running  at  a  great 
velocity  produces  a  movement  in  the  air  which  may  be  some- 
times usefully  and  economically  employed  for  ventilating 
tunnels.  Water  falling  vertically  is  let  run  into  a  large 
horizontal  zinc  pipe  having  a  funnel  at  the  outer  end ;  into  this 
the  air  attracted  by  the  velocity  of  the  water  is  forced.  By  an 
opening  at  the  bottom  the  water  is  afterward  withdrawn  from 
the  pipe,  and  there  remains  only  the  air  which  is  pushed  for- 
ward by  the  air  which  is  being  continually  sucked  in  by  the 
velocity  of  the  water. 

The  best  and  most  common  means  of  ventilation  by  the 
plenum  method  is  by  fans.  There  are  numerous  varieties  of 
these  fans  in  the  market,  but  they  all  consist  of  a  kind  of  fan 
wheel  which  by  rapid  revolution  forces  the  fresh  air  into  the 
pipe  leading  to  the  headings  of  the  tunnel  or  to  the  working 
places.  Instead  of  a  large  single  fan,  such  as  is  used  for  min- 


VENTILATION    AND   LIGHTING   DURING    CONSTRUCTION       295 

ing  purposes,  it  is  better  to  have  a  number  of  small  fans  acting 
independently  of  each  other,  conveying  tne  fresh  air  where  it  is 
needed  through  independent  pipes. 

Compressed  Air.  —  In  the  excavation  of  tunnels  in  hard  rock 
a  number  of  rock  drilling  machines  are  employed  which  are 
moved  by  compressed  air  at  a  pressure  of  not  less  than  frve 
atmospheres.  At  each  stroke  about  100  cu.  ins.  of  compressed 
air  is  set  free,  and  at  an  average  of  10  strokes  per  minute  there 
would  be  5,000  cu.  ins.  of  air  at  five  atmospheres  or  25,000 
cu.  ins.,  or  a  little  more  than  175  cu.  ft.  of  fresh  air  at  normal 
pressure  set  free  every  minute  by  each  of  the  machines  employed. 
It  may  be  assumed  that  in  a  long  tunnel  there  are  at  least  ten 
machines  at  work  at  the  same  time  at  different  points,  then  the 
quantity  of  fresh  air  thus  introduced  in  the  tunnel  may  be  cal- 
culated as  nearly  2,000  cu.  ft.  per  minute,  or  about  4,500  cu. 
yds.  per  hour. 

Regarding  ventilation  by  compressed  air,  Mr.  Adolph  Sutro, 
in  a  lecture  delivered  to  the  mining  students  of  the  University 
of  California,  said : 

"  I  will  note  a  curious  fact  which  I  have  never  seen  explained,  and  which  is 
worthy  of  close  investigation  by  means  of  experiments.  In  the  Sutro  tunnel 
we  found  that  the  compressed  air  used  for  driving  the  machine  drills,  after  hav- 
ing been  compressed  and  expanded  and  discharged  from  the  drills,  was  not 
wholesome  to  breathe,  and  the  men  and  mules  would  all  crowd  around  the  end 
of  the  blower  pipe  to  get  fresh  air.  Whether  the  air  in  being  compressed  has 
parted  with  some  of  its  oxygen  or  because  vitiated  from  some  other  cause,  I  do 
not  know,  and  1  hope  that  this  subject  will  at  some  future  day  be  carefully  ex- 
amined into." 

Quantity  of  Air The  quantity  of  air  to  be  introduced  into 

tunnels  must  be  in  proportion  to  the  oxygen  consumed  by  the 
men,  the  animals,  and  the  explosions.  It  is  allowed  that  the 
quantity  of  air  required  for  breathing  purpose  and  explosions  is 
as  follows : 

1  workman  with  lamp  needs  240  cu.  yds.  of  fresh  air  in  24  hours 
1  horse  ««      850       "  "       "  " 

1  Ib.  gunpowder  100      "  "      " 

1  Ib.  dynamite  150       "  "       " 


296  TUNNELING 

In  a  long  tunnel  excavated  through  hard  rock  the  number 
of  workmen  all  together  may  be  assumed  at  400  at  each  end, 
and  each  workman  is  supposed  to  be  furnished  with  a  lamp. 
No  less  than  ten  horses  are  employed,  and  the  average  quantity 
of  dynamite  consumed  is  600  Ibs.  per  day.  From  the  data  given 
the  consumption  of  air  by  workmen  and  lamps  would  be : 
240x400=96,000  cu.  yds. ;  the  consumption  of  air  by  horses 
would  be  850  x  10  =  8,500  cu.  yds. ;  the  consumption  of  air  by 
dynamite  would  be  150  x  600  =  99,000  cu.  yds. ;  making  a  total 
consumption  of  air  per  day  of  197,500  cu.  yds.,  or  about  8,000 
cu.  yds.  per  hour. 

To  obtain  good  ventilation,  then,  it  will  be  necessary  to 
furnish  every  hour  a  quantity  of  fresh  air  amounting  to  not  less 
than  8,000  cu.  yds.  Since,  however,  a  large  quantity  of  pure 
air  is  expelled  with  the  foul  air,  it  is  necessary  greatly  to  in- 
crease this  quantity. 

It  may  be  observed,  in  closing,  that  the  water  having  its 
particles  divided,  as  in  a  fog  or  mist,  rapidly  precipitates  the 
gases  produced  by  explosions.  Now,  when  hydraulic  machines 
are  used,  there  is  a  hollow  ball  pierced  by  holes  that  are  almost 
imperceptible,  from  which  the  compressed  water  spreads  in  very 
subtile  particles,  and  this  causes  the  fall  of  the  gases  from 
explosions.  Such  a  method  of  precipitating  gases  is  very  good, 
but  does  not  have  the  advantage  of  supplying  new  oxygen  to 
replace  that  consumed  by  the  men,  animals,  lamps,  and  ex- 
plosions ;  besides,  it  has  the  defect  of  increasing  the  quantity  of 
water  to  be  removed.  In  tunnels  the  pipes  used  either  for  con- 
veying the  fresh  air  or  for  carrying  away  the  foul  air,  are  of 
iron,  having  a  diameter  of  about  8  in. ;  they  are  fixed  along  the 
side  walls  about  3  ft.  above  the  inverted  arch. 


VENTILATION    AND    LIGHTING   DURING    CONSTRUCTION       297 


LIGHTING. 

The  object  and  necessity  of  a  perfect  lighting  of  the  tunnel- 
workings  during  construction  are  so  obvious  that  they  need  not 
be  enlarged  upon.  Comparatively  few  tunnels  require  lighting 
after  completion  ;  and  these  are  generally  tunnels  for  passenger 
traffic  under  city  streets,  of  which  the  Boston  Subway  is  a  rep- 
resentative American  example.  Considering  the  methods  of 
lighting  tunnels  during  construction,  we  may,  for  sake  of  con- 
venience, chiefly,  divide  the  means  of  supplying  light  into  (1) 
lamps  and  lanterns  usually  burning  oil ;  (2)  coal-gas  lighting  ; 
(3)  acetylene  gas  lighting;  and  (4)  electric  lighting. 

Lamps  and  Lanterns.  —  Lamps  and  lanterns  are  commonly 
employed  by  engineers  for  making  surveys  inside  the  tunnel,  and 
to  light  the  instrument  For  ranging  in  the  center  line,  a  con- 
venient form  of  lamp  consists  of  an  oil  light  inclosed  in  glass 
chimney  covered  with  sheet  metal,  except  for  a  slit  at  the  front 
and  back  through  which  the  light  shines,  and  on  which  the 
observer  sights  his  instrument.  To  direct  the  operations  of  his 
rodmen  the  engineer  usually  employs  a  lantern,  either  with 
white  or  colored  glass,  much  like  the  ordinary  railway  train- 
man's lantern,  which  he  swings  according  to  some  prearranged 
code  of  signals. 

Lamps  and  lanterns  are  used  by  the  workmen  both  for  sig- 
naling and  for  lighting  the  woi kings.  For  signaling  purposes 
red  lanterns  are  usually  placed  to  denote  the  presence  of  unex- 
ploded  blasts  or  other  points  of  possible  danger ;  and  colored  or 
white  lights  are  usually  placed  on  the  front  and  rear  of  spoil 
and  material  trains.  For  lighting  purposes,  two  forms  of  lamps 
are  employed,  which  may  be  somewhat  crudely  designated  as 
lamps  for  individual  use  and  lamps  for  general  lighting.  Indi- 
vidual lamps  are  usually  of  small  size,  and  burn  oil ;  they  may 
be  carried  in  front  of  the  miner's  helmet,  or  be  fixed  to  stand- 
ards, which  can  be  set  up  close  to  the  work  being  done  by  each 


298  TUNNELING 

man.  Miners'  safety  lamps  should  be  employed  where  there  is 
danger  from  gas.  A  great  variety  of  lamps  for  mining  and 
tunneling  purposes  are  on  the  market,  for  descriptions  of 
which  the  reader  is  referred  to  the  catalogues  of  their  manu- 
facturers. 

Lamps  for  general  lighting  are  always  of  larger  size  than 
lamps  for  individual  use.  A  common  form  consists  of  a  cyl- 
inder ten  or  twelve  inches  in  diameter,  provided  with  a  hook  or 
bail  for  suspension,  and  filled  with  benzine,  gasolene,  or  other 
similar  oil.  Connected  with  this  cylinder  is  a  pipe  of  con- 
siderable length  and  small  diameter  through  which  the  benzine 
or  gasolene  vapor  runs,  and  burns  when  lighted  with  a  brilliant 
flame.  Lamps  of  this  type  burning  gasolene  were  extensively 
employed  in  building  the  Croton  Aqueduct  Tunnel.  Various 
patented  forms  of  lamps  for  burning  coal-oil  products  are  on 
the  market,  for  descriptions  of  which  the  manufacturers'  cata- 
logues may  be  consulted. 

Coal-gas  Lighting.  —  A  common  method  of  lighting  tunnel 
workings  is  by  piping  coal-gas  into  the  headings  and  drifts  from 
some  nearby  permanent  gas  plant,  or  from  a  special  gas  works 
constructed  especially  for  the  work.  Gas  lighting  has  the  great 
advantage  over  lamps  and  lanterns  of  giving  a  light  which  is 
more  brilliant  and  steady.  Its  great  objection  is  the  danger  of 
explosion  caused  by  leaks  in  the  pipes,  by  breaks  caused  by 
flying  fragments  of  rock,  and  by  the  carelessness  of  workmen 
who  neglect  to  turn  off  completely  the  burners  when  they  ex- 
tinguish the  lights.  In  nearly  every  tunnel  where  gas  has  been 
used  for  lighting,  the  records  of  the  work  show  the  occurrence 
of  accidents  which  have  sometimes  been  very  serious,  partic- 
ularly when  fire  has  been  communicated  to  the  tunnel  tim- 
bering. 

Acetylene  Gas  Lighting.  —  The  comparatively  recent  devel- 
opment of  acetylene  gas  manufactured  from  carbide  of  calcium 
has  given  little  opportunity  for  its  use  in  tunnel  lighting,  and 
the  only  instance  of  its  use  in  the  United  States,  so  far  as  the 


VENTILATION   AND   LIGHTING    DURING    CONSTRUCTION       299 

author  knows,  is  the  water-works  tunnel  conduit  for  the  city  of 
Washington,  B.C.  Col.  A.  M.  Miller,  U.S.  Engineer  Corps, 
who  is  in  charge  of  this  work,  desciibes  the  method  adopted  in 
his  annual  report  for  1899  as  follows :  — 

"It  had  been  the  practice  to  do  all  work  underground  by  the  light  of 
miners'  lamps  and  torches.  This  means  of  illumination  is  very  poor  for  me- 
chanical work.  The  fumes  and  smoke  from  blasting,  added  to  the  smoke  from 
torches  and  lamps,  render  the  atmosphere  underground,  especially  when  the 
barometer  conditions  were  unfavorable  to  ventilation,  very  offensive  and  dis- 
comforting to  the  workmen.  An  investigation  of  the  subject  of  lighting  the 
tunnel  by  other  means,  more  especially  at  the  locality  where  the  mechanics 
were  at  work,  — brick  and  stone  masons,  and  the  workmen  on  the  iron  lininp, 
—  resulted  in  the  selection  of  acetylene  gas  as  the  most  available  and  economi- 
cal in  this  special  emergency.  Accordingly,  an  acetylene  gas  plant  for  SCO 
burners  was  erected  at  Champlain- Avenue  shaft,  and  one  for  60  lights  at  Foun- 
dry Branch.  The  engine-houses  at  the  shafts,  the  head-houses,  and  localities 
in  the  tunnel,  when  required,  are  lighted  by  these  plants. 

"  Gas  pipes  were  carried  down  the  Champlain- A  venue  shaft  and  along  the 
tunnel  both  in  an  easterly  and  westerly  direction,  with  cocks  for  burners  at 
proper  intervals  every  30  feet ;  and  this  system  sufficed  for  illumination  from 
Rock  Creek  to  Harvard  University,  a  distance  of  over  two  miles.  The  plant 
erected  at  Foundry  Branch  was  in  like  manner  utilized  for  the  illumination 
from  that  point  in  both  directions. 

"By  connecting  with  the  stopcocks  by  means  of  a  rubber  hose,  a  movable 
light,  chandelier,  or  '  Christmas-tree '  of  any  required  number  of  burners  is 
used,  thus  concentrating  the  light  in  the  immediate  vicinity  of  the  work,  and 
also  enabling  the  illumination  to  be  carried  into  the  cavities  or  'crow-nests,'  so- 
called,  behind  the  defective  old  lining. 

"This  method  of  illuminating  has  proved  very  satisfactory  and  quite  eco- 
nomical. It  is  especially  valuable  as  enabling  good  work  to  be  done,  and 
facilitating  a  thorough  inspection  of  the  same." 

Electric  Lighting.  —  By  far  the  most  perfect,  and  at  present 
the  most  commonly  employed  means  for  lighting  tunnel  work- 
ings, is  electricity.  The  light  furnished  by  electric  lamps  is 
steady  and  brilliant,  and  does  not  consume  oxygen  or  give  off 
offensive  gases.  The  wires  are  easily  removed  and  extended, 
and  the  lamps  are  easily  put  in  place  and  removed.  About  the 
only  objection  to  the  method  is  the  fragility  of  the  lamps,  which 
are  easily  broken  by  the  flying  stones  and  the  concussion  pro- 
duced by  blasting. 


300  TUNNELING 


CHAPTER  XXV. 

THE    COST    OF    TUNNEL    EXCAVATION   AND 
THE    TIME    REQUIRED    FOR    THE    WORK. 


Cost. THE  cost  of  a  tunnel  will  depend  upon  the  cost  of 

the  two  principal  operations  required  in  its  construction,  viz., 
the  excavation  of  the  cross  section  and  the  lining  of  the  exca- 
vation with  masonry,  metal,  or  timber.  These  two  operations 
may  in  turn  be  subdivided,  in  respect  to  expense,  into  cost  of 
labor  and  cost  of  materials.  It  is  a  comparatively  simple  mat- 
ter to  calculate  the  cost  of  the  building  materials  required  to 
construct  a  tunnel ;  but  it  is  veiy  difficult  to  estimate  with 
accuracy  what  the  cost  of  labor  will  be.  The  reason  for  this  is 
that  it  is  impossible  to  foresee  exactly  what  the  conditions  will 
be  ;  the  character  of  the  material  may  change  greatly  as  the 
work  proceeds,  increasing  or  decreasing  the  cost  of  excavation  ; 
water  may  be  encountered  in  quantities  which  will  materially 
increase  the  difficulties  of  the  work,  etc.  Nevertheless,  while 
accurate  preliminary  estimates  of  cost  are  not  practicable,  it  is 
always  desirable  to  attempt  to  obtain  some  idea  of  the  probable 
expense  of  the  work  before  beginning  it,  and  the  more  usual 
means  of  getting  at  this  point  will  be  discussed  here. 

Two  methods  of  estimating  the  cost  of  tunnel  work  are  em- 
ployed. The  first  is  to  calculate  the  probable  expense  of  the 
various  items  of  work,  based  upon  the  available  data,  per  unit 
of  length,  and  then  add  to  this  a  margin  of  at  least  1 0  %  to  allow 
for  contingencies ;  the  second  is  to  apply  to  the  new  work  the 
unit  cost  of  some  previous  tunnel  built  under  substantially  the 
same  conditions.  In  the  first  method  it  is  usual  to  consider 
the  strutting  and  hauling  as  constituting  a  part  of  the  work  of 


COST   OF    EXCAVATION   AND   TIME   REQUIRED  301 

excavation.  To  estimate  the  cost  of  excavation  involves  the 
consideration  of  three  general  items,  viz.,  the  excavation  proper, 
the  strutting  of  the  walls  of  the  excavation,  and  the  hauling  of 
the  excavated  materials  and  the  materials  of  construction. 

The  cost  of  excavating  the  preliminary  headings  or  drifts  is 
greater  per  unit  of  material  removed  than  that  of  excavating 
the  enlargement  of  the  section.  The  cost  of  bottom  drifts  is 
also  always  greater  than  that  of  top  headings,  the  material  pene- 
trated remaining  the  same.  Mr.  Rziha  gives  the  comparative 
unit  costs  of  excavating  drifts,  headings,  and  enlargement  of 
the  profile  as  follows :  — 

Bottom  drifts $9.20  per  cu.  yd. 

Top  headings 4.80     "     "     " 

Enlargement  of  profile 2.84     "     **     " 

The  cost  of  hauling  increases  with  the  length  of  the  tunnel. 
This  fact  and  amount  of  this  increase  are  indicated  by  the 
following  actual  prices  for  the  Arlberg  tunnel :  — 

Top  heading 86.76  per  cu.  yd.,  increasing  37  cts.  per  mile 

Bottom  drift 7.40     "     "      "  "          26  "      "       " 

Enlargement  of  profile     .     .     .       2.70     "     "      ««  "  10  "      "       " 

In  all  the  prices  given  above,  the  cost  of  strutting  and  haul- 
ing is  included  in  the  cost  of  excavation. 

The  cost  of  excavation  is  not  always  the  same  for  the  same 
character  of  materials  in  different  tunnels.  The  following 
figures  show  the  prices  paid  for  the  excavation  of  calcareous 
rock  in  four  different  German  tunnels :  — 

Berliner  Nordhausen  Wetzler  R.R.  ;     .     .     .  $1.24  per  cu.  yd. 

Open 1.30     "     "       " 

Stafflach ,     .     .     .  2.76     "     "       " 

Gries 1.92     "     "       " 

The  method  of  tunneling  has  little  influence  upon  the  cost 
of  the  work,  as  shown  by  the  following  figures  from  tunnels 
excavated  through  calcareous  rock  by  different  methods :  — 


302  TUNNELING 

Open  tunnei  Austrian  method '$93.19  per  lin.  ft. 

Dorremberg  tunnel        Belgian  method 86.08     "     '•     '• 

Stafflach  tunnel  English  method 91.69     "     "     " 

The  Martha  and  Merten  tunnels,  excavated  through  soft 
ground  by  the  Austrian  and  German  methods  respectively, 
cost  187.95  and  187.55  per  lin.  ft.  respectively.  In  the  exca- 
vation of  the  various  sections  of  the  tunnel  for  the  new  Croton 
Aqueduct  in  America,  the  following  prices  were  paid :  - 

Excavation  of  heading       ......  $8  to  $10.00  per  cu.  yd. 

Tunnel  in  soft  ground 8  to      9.00     "     "      " 

Tunnel  in  rock 7  to      8.50     "     "      " 

Brick  masonry 10.00     "     "      " 

Timber  in  place $40  per  M.  ft.  B.  M. 

It  is  the  practice  in  America  to  include  the  work  of  hauling 
under  excavation,  but  not  to  include  the  strutting,  which  is 
paid  for  separately.  In  some  cases  only  the  market  price  of 
the  timber  is  paid  for  separately,  the  cost  of  setting  up  being 
included  in  the  price  of  excavation.  The  writer  prefers  the 
European  practice  of  including  the  total  cost  of  timbering 
under  excavation,  since  the  two  operations  are  so  closely  con- 
nected, and  since  the  contractor  employs  the  same  timber  over 
and  over  again.  Knowing  the  dimensions  of  the  several  mem- 
bers of  the  strutting,  it  is  a  simple,  although  somewhat  tedious, 
process  to  calculate  the  total  quantity  required.  An  idea  of 
the  quantity  of  timber  required  for  strutting  in  soft  ground 
may  be  had  from  the  data  given  on  page  50.  The  quantity 
will  decrease  as  the  cohesion  of  the  material  penetrated  in- 
creases, until  it  becomes  so  small  in  hard  rock-tunnels  as  to  cut 
very  little  figure  in  the  total  cost. 

The  cost  of  hoisting  excavated  materials  through  shafts 
depends  upon  the  depth  from  which  it  is  hoisted,  and  upon  the 
character  of  hoisting  apparatus  employed.  The  following  table, 
showing  the  cost  of  hoisting  for  different  lifts  and  by  different 
methods,  is  given  by  Rziha,  the  cost  being  in  francs  per  cubic 
meter :  — 


COST    OF    EXCAVATION    AND    TIME    REQUIRED 


303 


WlXDLASS. 

HOKSK  Gixs. 

STEAM  HOISTS. 

METRES. 

Francs  per  Cu.M. 

ONE  HORSE. 
Francs  per  Cu.  M. 

Two  HORSES. 
Francs  per  Cu.  M. 

Francs  per  Cu.  M. 

15 

0.172 

0.077 

0.062 

0.035 

30 

0.212 

0.087 

0.070 

0.045 

45 

0.257 

0.100 

0.080 

0.050 

60 

0.305 

0.112 

0.092 

0.082 

90 

0.410 

0.152 

0.110 

0.087 

120 

0.535 

0.195 

0.135 

0.092 

150 

0.722 

0.240 

0.157 

0.112 

Mr.  Sejourne,  a  French  engineer,  who  has  been  connected 
with  the  construction  of  numerous  tunnels  by  the  Belgian 
method  where  he  was  in  position  to  secure  comparative  figures, 
has  given  the  following  rules  for  calculating  the  cost  of 
tunnels.  Assuming  A  to  represent  the  cost  of  excavating  a 
cu.  yd.  in  the  open  air,  the  cost  of  excavating  the  same 
quantity  underground  in  driving  headings  will  be  from  9  A.  to 
11  A,  and  in  enlarging  the  profile  it  will  be  about  5  A.  The 
cost  of  constructing  single-track  tunnels  varies  with  the  thick- 
ness of  the  lining,  and  may  be  calculated  by  the  following 
formulas : 

Without  lining,  C  =  5. 5  A. 

With  roof  arch  only,          C  =  6.4  A  +  64. 
With  lining  18  in.  thick,  C  =  9.4  +  1A. 
With  lining  2  ft.  thick,      C  =  11  +  8  A. 

In  these  formulas  0  is  the  cost  per  cu.  yd.  of  excavation, 
including  the  masonry.  For  double-track  tunnels  the  amounts 
given  by  the  above  formulas  may  be  used  by  reducing  them 
about  ll  %  or  8  %. 

The  second  method  of  estimating  the  cost  of  tunnel  work 
consists  in  assuming  as  a  unit  the  unit  cost  of  tunnels  pre- 
viously excavated  under  similar  conditions.  Mr.  La  Dame 
gives  the  following  unit  prices  for  a  number  of  tunnels  driven 
through  different  materials : 


304 


TUNNELING 


NATURE  OF  SOIL. 

3*0 
flO 

H 

EXCAV.   PER 

Cu.  YD. 

COST  PER 
LIN.  FT. 

MAX.   AND   MlN. 

PER  LIN.  FT. 

Granite-gneiss      .     . 
Schist     

56 
8Q 

$3.07  @$3.85 
1.38  @    1.53 

$100. 
75.42 

$61.46    @$190.40 
43.11     @      70.68 

Triassic  
Jurassic      .... 
Cretaceous  .... 
Tertiary  and  modern 

3 

69 
34 
39 

1.23  @    L38 
0.61  @    0.77 
0.33  @    0.61 

90.85 
77.86 
59.60 
105.80 

84.75    @     93.33 
35.24     @    157.2 
27.37     @     92.25 
51.52     @    188.36 

In  the  following  table  is  given  a  list  of  tunnels  excavated 
through  different  soils,  from  the  most  compact  to  very  loose 


DOUBLE-TRACK  TUNNELS. 


NAME  OF  TUNNELS. 

QUALITY  OF  SOIL. 

COST  PER 
LIN.  FT. 

METHOD  OF 
TUNNELING. 

Mt.  Cenis     .... 

Granitic 

$273  73 

Drift. 

St.  Gothard  

193  63 

Heading 

Stammerich  

Granitic 

157  90 

English. 

Stalle  

Broken  schist, 

290  58 

Austrian. 

Bothenfels    

Dolomite, 

115  64 

English. 

Dorremberg      .... 
Stafflach  .     . 

Calcareous, 
Calcareous 

86.08 
91  69 

Belgian. 
English 

Open  

Calcareous 

93  19 

Austrian 

Wartha    

Grevvack 

87  95 

Austrian 

Mertin     

Grewack 

87  55 

German  . 

Scloss  Matrei    .... 
Trietbitte     

Clay  schist, 
Clay  and  sand, 

94.25 
229  0 

English. 
German. 

Canaan    

Clay-slate, 

69  50 

Wide  heading. 

Church-Hill      .... 
Bergen  No.  1    .... 

Clay  with  shells, 
Trap  rock, 

178.0 
182.31 

SINGLE-TRACK  TUNNELS. 


NAME  OF  TUNNELS. 

QUALITY  OF  SOIL. 

COST  PER 
LIN.  FT. 

METHOD  OF 
TUNNELING. 

Mt.  Cenis    .... 

Gneiss 

382  27 

Headin0" 

Stalletti  

Granite  and  quartz 

62  75 

Austrian 

Marein    

Clay  schist 

64  36 

English 

Welsberg     

Gravel 

165  07 

Austrian 

Sancina  

Clay  of  1st  variety 

129  40 

Belgian 

Starre 

Clay  of  2d  variety 

191  61 

Cristina  . 

Clay  of  3d  variety 

307  42 

Burk  

83  90 

W^ide  headin0" 

Brafford  Ridge  .     .     .     . 

85  33 

Wide  headinjr 

Dunbeithe    

Limestone 

70  47 

Fergusson    

Sandstone 

37  46* 

Port  Henry  
Points      .     .     . 

Limestone, 

80.001 

72  00* 

Wide  heading. 

Wirlp   liPQrlinrr 

*  Are  unlined. 


t  Lined  with  timber. 


COST   OF   EXCAVATION   AND   TIME   REQUIRED  305 

materials,  and  driven  according  to  the  various  methods  which 
have  been  illustrated. 

The  Habas  tunnel  through  quicksand,  between  Dax  and 
Ramoux,  France,  cost  1118.50  per  lin.  ft.  The  cost  of 
the  Boston  subway  was  $342.40  per  lin.  ft.  The  Severn 
and  Mersey  tunnels,  constructed  through  rock  under  water, 
cost  respectively  $208.33  and  $263  per  lin.  ft.  The  First 
Thames  Tunnel,  driven  by  Brunei's  shield,  cost  $1661.66  per 
lin.  ft.  The  Hudson  River  and  St.  Clair  River  tunnels,  exca- 
vated through  soft  ground  by  means  of  shields  and  compressed 
air,  cost  respectively  $305  and  $315  per  lin.  ft.  The  Black- 
wall  double-track  tunnel  under  the  River  Thames,  which  is 
the  largest  tunnel  ever  built  by  the  shield  system,  cost  $400 
per  lin.  ft. 

In  making  estimates  of  the  cost  of  projected  tunnel  work 
based  on  the  cost  of  tunnels  previously  constructed  through 
similar  materials,  it  is  important  to  keep  in  mind  the  date  and 
location  of  the  work  used  as  the  basis  for  calculations.  For 
example,  a  tunnel  excavated  in  Italy,  where  labor  is  very  cheap, 
will  cost  less  than  one  excavated  in  America,  where  labor  is 
dear,  all  other  conditions  being  the  same.  Other  reasons  for 
variation  in  cost  due  to  difference  of  date  and  location  of  con- 
struction will  suggest  themselves,  and  should  be  taken  into  full 
consideration  in  estimating  the  cost  of  the  new  work. 

Time.  —  The  time  required  to  excavate  a  tunnel  depends 
upon  the  character  of  the  material  penetrated  and  upon  the 
method  of  work  adopted.  Tunnels  driven  through  soft  ground 
by  hand  require  about  the  same  time  to  construct  as  tunnels 
driven  through  hard  rock  by  the  aid  of  machinery.  Tunnels 
can  be  driven  through  hard  rock  at  about  as  great  a  speed  as 
through  soft  or  fissured  rock,  chiefly  because  the  work  of 
blasting  is  more  efficient  in  hard  rock,  and  because  no  time 
is  required  in  timbering.  The  following  table  shows  the 
average  rate  of  progress  in  different  parts  of  the  tunnel  excava- 
tion through  both  hard  and  soft  materials  in  feet  per  month :  — 


306 


TUNNELING 


QUALITY  OF  SOIL. 

HEADING. 

EXCAVATION  OF  SHAFTS. 

ENLARGE- 
MENT OF 
PROFILE. 

By  hand. 

By  machine. 

By  hand. 

By  machine. 

By  hand. 

Very  loose  soil  . 
Loose  soil    .  .     . 
Soft  rock  . 
Hard  rock     . 
Very  hard  rock, 

16.7  -26.8 
33.4  -100 
66.8 
50      -66.8 
33.4 

233.8-334 
233.8-334 
233.8-334 

6.6-16.7 
16.7-33.4 
33.4-66.8 
33.4-50 
16.7-33.4 

66.8-L32.6 
66.8-132.6 
66.8-132.6 

6.6-16.7 
16.7-33.4 
33.4-50 
66.8-100 
66.8-100 

The  following  tables  showing  the  average  rate  of  progress 
have  been  compiled  from  the  actual  records  made  in  the 
tunnels  named : 


NAME  OF  TUNNEL. 

DIMENSIONS 
IN  FEET. 

MONTHLY 
PROGRESS 
IN  FEET. 

CHARACTER  OF 
MATERIAL. 

OBSERVATIONS. 

Excavation  of  headings 

by  hand: 

Mount  Cenis 

10X10 

65.8 

Schist, 

Bottom  drift. 

Sutro  

6.7  X5.7 

70.14 

Quartzose, 

St.  Gothard  .     .     . 

8.4X8.7 

70.14 

Granite, 

Top  heading. 

Excavation  of  headings 

by  machine: 

Mount  Cenis      .     . 

10  x  10 

188.7 

Calcareous  schist, 

Bottom  drift. 

Sutro  ... 

8.15X10 

227.45 

Quartzose, 

. 

St.  Gothard  .     .     . 

8.4X8.7 

339.45 

Granite, 

Top  heading. 

Trari    

8  X  9.35 

167 

Gneiss, 

Top  heading. 

Arlberg    .... 

8.35  x  9.35 

474.2 

Mica  schist, 

Bottom  drift. 

Palisades  .... 

16X7 

160 

Trap  rock, 

Top  heading. 

Busk    

15x7 

126 

Granite, 

Top  heading. 

Cascade    .... 

16  x  8 

180 

Basaltic  rock, 

Top  heading. 

Franklin  .... 

15  x  7 

240 

Top  heading. 

The  following  table  shows    the    monthly  progress  of  com- 
pleted tunnel  in  feet  excavated  through  rock : 


NAME  OF  TUNNEL. 

PROGRESS 
IN  FEET. 

MATERIAL. 

METHOD. 

Cascade    

207 

Basalt, 

Top  heading 

Palisades       .... 

186 

Trap  rock 

Top  heading 

Busk    

190 

Granite 

Top  heading 

Tennessee  Pass  

169.6 

Granite 

Top  heading 

COST   OF   EXCAVATING   AND   TIME   REQUIRED 


307 


The  average  monthly  progress  in  feet  of  excavating  tunnels 
through  treacherous  ground  may  be  quite  generally  assumed 
to  be  for:  (1)  clay  of  the  first  variety  from  43.4  ft.  to  60  ft. ; 
for  clay  of  the  second  variety  from  33.4  ft.  to  43.4  ft. ;  for  clay 
of  the  third  variety  from  23.3  ft.  to  33.4  ft.,  and  for  quicksand 
from  30  ft.  to  50  ft.  The  monthly  progress  in  feet  made  in 
sinking  the  shafts  of  the  Hoosac  and  Musconetcong  tunnels  in 
America  was  as  follows  :  — 


NAME  OF  TUNNEL. 

DIMENSIONS 
IN  FEET. 

DEPTH 
IN  FEET. 

PROGRESS 
IN  FEET. 

CHARACTER 
OF  MATERIAL. 

Hoosac: 

East  shaft  

15.4  X  27.7 

1035 

21.7 

Mica  schist. 

West  shaft     

8X  16 

267 

16.7 

Gneiss. 

Musconetcong: 

Vertical  shaft     .... 

8.35  X  16.7 

113.5 

100 

Loose  rock. 

Inclined  shaft     .... 

8.35  x  26 

304. 

32 

Loose  rock. 

The  average  monthly  progress  of  sinking  shafts  in  treach- 
erous soils  may  be  assumed  to  be  as  follows:  clay  of  first 
variety,  50  ft.  to  75  ft. ;  clay  of  second  variety,  36.75  to  50  ft ; 
clay  of  third  variety,  23.4  ft.  to  36.75  ft ;  quicksand,  16.7  ft. 
to  33.4  ft. 


INDEX 


PAGE       Excavation :  (continued) 


Air  Compressors,  Description  of  .    . 

Accidents  in  Tunnels 142, 154 

After  Construction 273 

Chattanooga  Tunnel 276 

During  Construction 266 

Giovi  Tunnel 274 

Repairing  of 269 

Baltimore  Belt  Line  Tunnel,  Gen- 
eral Description 150 

Boston  Subway,  General  Description  .  187 

Busk  Tunnel,  General  Description  .    .  119 

Cascade  Tunnel 129 

Center  Line,  Determination  oi      .    .    .  9 

Curvilinear  Tunnels      ....  13 

Rectilinear  Tunnels 9 

Simplon  Tunnel 276 

Chattanooga  Tunnel,  Accident  in     .    .  276 

Cross-Section  : 

Boston  Subway 187 

Dimensions  of 17 

Form  of .-.    .  15 

New  York  Rapid  Transit  Railway 

Tunnel 195 

Croton  Aqueduct  Tunnel 126 

Culverts 75 

Drills : 

Hand 20 

Power 21,84 

Percussion 21 

Mountings  for 22 

Rotary  . 22, 102 

East   River  Gas    Tunnel,  General 

Description 208 

Entrances 77 

Excavation : 

Austrian  method 162 

Advantages  and  Disadvantages  166 

Baltimore  Belt  Line  Tunnel   ...  151 

Belgian  Method 135 

Advantages  and  Disadvantages  142 

Boston  Subway 188 


PAGE 


Busk  Tunnel 

Cost  of 

Division  of  Section 

English  Method 

Advantages  and  Disadvantages 

Enlargement  of  Profile 

German  Method 

Advantages  and  Disadvantages 

Headings 

Italian  method 

Advantages  and  Disadvantages 

Mont  Cenis  Tunnel 

Rock,  Methods  of 

Quicksand  Method 

Simplon  Tunnel 

Tunnels,  open-cut : 

Parallel  Longitudinal  Trenches 

Single  Longitudinal  Trenches  . 

Transverse  Trenches    .... 
Excavators  : 

Machines  for  Earth 

Machines  for  Rock 

Explosives : 

Dynamite 

Gunpowder 

Nature  of 

Nitroglycerine 

Storage  of 

Use  in  Blasting 


300 
32 
156 
161 
34 
145 
149 
33 
167 
173 


174 

100 

181 
180 
182 

19 


Fuses 


Geological  Surveys : 

Method  and  Purpose  of 
Simplon  Tnnnel     .    .    . 
Graveholz  Tunnel   . 


Hauling : 

Belgian  Method  .  .  . 
By  Way  of  Entrances 
By  Way  of  Shafts  .  . 

Definition 

English  Method  .  .  , 
German  Method  .  . 
Italian  Method  .  .  , 


27 


3 
96 
129 


140 
55 
58 
55 
161 
148 
170 


309 


310 


INDEX 


Hauling :  (continued)  PAGE 

Mont  Cenis  Tunnel 92 

Saint  Gothard  Tunnel 119 

Simplon  Tunnel 102 

Hoisting  Machinery       58 

Hoosac  Tunnel      124 

Jacks 263 

Lighting : 

Acetylene  Gas 298 

Coal  Gas 298 

Electricity 299 

Lamps  and  Lanterns 297 

Linings  : 

Iron 69,215 

Iron  and  Masonry  : 70 

New  York  Kapid  Transit  Ry. 

Tunnel 196 

Masonry  : 70 

Accidents  and  Repairs  to      .    .  142 

Austrian  Method 165 

Baltimore  Belt  Line  Tunnel     .  152 

Belgian  Method 137 

Boston  Subway 190 

Centers  for  Arches 64 

English  Method 159 

German  Method 148 

Ground  Molds  for 62 

Invert 73 

Italian  Method 169 

Leading  Frames  for 63 

Mont  Cenis  Tunnel 92 

Quicksand  Method 176 

Hoof  Arch 72 

Saint  Gothard  Tunnel  ....  117 

Side  Walls 72 

Thickness  of 74, 78 

Purpose  of 68 

Timber 68 

Machinery,  Hoisting 58 

Materials,  Character  of : 3 

Baltimore  Belt  Line  Tunnel  ...  150 

Boston  Subway 187 

Busk  Tunnel 119 

Mont  Cenis  Tunnel 88 

New  York  Kapid  Transit  Tunnel    .  194 

Saint  Gothard  Tunnel 116 

Milwaukee  Water-works  Tunnel  ...  230 
Mont  Cenis  Tunnel,  General  Descrip- 
tion         ...  87 

New  York  Rapid  Transit  Railway, 

Tunnel,  General  Description  ...  192 

Niagara  Falls  Power  Tunnel    ....  128 

Niches 76 


PAGE 

Open-cut,  Choice  Between  a  Tunnel 

and l 

Palisades  Tunnel 125 

Power-Plants  : 

Air  compressors 81 

Busk  Tunnel  . 121 

Canals  and  Pipe  Lines 81 

Cascade  Tunnel 129 

Croton  Aqueduct  Tunnel    ....  126 

General  Description 79 

Graveholz  Tunnel 129 

Hoosac  Tunnel 124 

Mont  Cenis  Tunnel 90 

Niagara  Falls  Power  Tunnel  .    .    .  128 

Palisades  Tunnel 125 

Receivers ,     .  84 

Reservoirs .    .  81 

Saint  Clair  River  Tunnel    ....  130 

Saint  Gothard  Tunnel 117 

Simplon  Tunnel 108 

Sonnstein  Tunnel 130 

Steam 80 

Strickler  Tunnel 127 

Turbines 81 

Saint  Clair  River  Tunnel    ....  130 
Saint    Gothard    Tunnel,  General  De- 
scription        114 

Severn  Tunnel.  General  Description    .  204 

Shafts,  Description  of .  36 

Shield  Construction  : 

Cellular  Division .258 

Diaphragm 259 

Front  End 257 

General  Form 255 

Jacks 263 

Rear  End 261 

Shell 256 

Shields  : 

Barlow's 246 

Blackwall  Tunnel 252 

Boston  Subway 255 

Broadway  Pneumatic  Railway  Tun- 
nel       249 

City  and  South   London  Railway 

Tunnel ,250 

Clichy  Sewer  Tunnel 25$ 

East  River  Gas  Tunnel 218 

First  Thames  Tunnel 245 

North  and  South  Woolwich  Subway  249 

Saint  Clair  River  Tunnel     ....  251 

Simplon  Tunnel,  General  Description  .  94 

Sonnstein  Tunnel . 130 

Strata,  Inclination  of & 

Strickler  Tunnel 127 


INDEX 


311 


Strutting :  PAGE 

Austrian  Method 163 

Baltimore  Belt  Line  Tunnel  ...  151 

Belgian  Method .     , 136 

English  Method  ........  157 

German  Method 146 

Iron: 

Full  Section.    .......  52 

Headings 52 

Shafts 53 

Italian  Method 168 

Mont  Cenis  Tunnel 91 

Quicksand  Method 175 

Saint  Gothard  Tunnel  .    .    .    .  116 
Timber : 

Dimensions .  50 

Face  of  Excavation 47 

Full  Section 47 

Headings  ..........  44 

Quantity .  50 

Shafts c  48 

Tamping      .    .     = *  29 

Timbering  (See  Strutting). 
Tunnels  : 

Classification  of  ........  38 

Hard  Rock    ........  39 

Loose  Soil .  39 

Open-cut 40 

Quicksand 40 

Submarine «  41 

Historical  Development      .    .    .    .  ix 

Open-cut,  General  Description   .    .  180 
Belining  of : 

Boulder  Tunnel ,  280 

Little  Tom  Tunnel    .....  286 

Mullan  Tunnel 284 


Tunnels  :  (continued)  PAGE 

Soft  Ground : 

Austrian  Method  .....  162 

English  Method t  156 

General  Discussion  .....  133 

German  Method 145 

Italian  Method 167 

Pilot  Method 177 

Quicksand  Method 173 

Submarine : 

East  River  Gas  Tunnel      ...  208 
General  Description     ....  201 
Milwaukee  Water-Works  Tun- 
nel    230 

Severn  Tunnel 204 

Shield  System,  History  of  De- 
velopment    242 

Van     Buren    Street    Tunnel, 

Chicago 225 

Surface 183 

Under  City  Streets  : 

Boston  Subway 186 

General  Discussion 184 

New  York  Rapid  Transit  Rail- 
way Tunnel     192 

Van  Buren  Street  Tunnel,  Chicago  225 

Ventilation  : 

Artificial 292 

Boston  Subway 191 

Mont  Cenis  Tunnel     ......  92 

Natural 291 

Plenum  Method .  294 

Simplon  Tunnel Ill 

Vacuum  Method     .......  293 

Water,  Presence  in  Tunnels    .    .    c    .  f 


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